There are a wide variety of methods to gain experience as a Disciple of the Hand, many of which can be performed at the same time. Understanding the mechanics behind all these methods will allow you to make the best use of your time and effort when leveling.
Base Experience
The experience yielded by completing a given recipe is heavily dependent on level. Recipes at or near your class level will yield large amounts of base experience, higher than defeating a monster of your level. Crafting is all about experience modifiers, especially the quality increase bonus that can quadruple your base experience, so a higher base experience is obviously better. That said, you will earn a smaller bonus on higher-level crafts due to the difficulty involved, so it's tough to determine exactly how much base experience is optimal.
As a general rule, crafting items is like defeating monsters in dungeons: it will make up a significant portion of your actual leveling, but because it doesn't yield huge chunks of experience at a time it can be hard to tell how significant it is.
Crafting recipes are organized by tiers of 5 levels. You will not be able to craft any recipe until you are within 3 levels of the minimum for that tier. Aside from that restriction, you are limited to crafting recipes as high as five levels above your current level. For instance, at level 37 your maximum recipe level is 40, since the 41-45 tier has not been unlocked. However, at level 38 that tier unlocks and you can immediately craft recipes up to level 43.
Completion Bonus Experience
Standard recipes and housing recipes offer a nice chunk of bonus experience the first time they are crafted. Once you've completed one of these recipes for the first time, a green check mark will appear on top of the item icon in your crafting log. This indicates that the bonus has been earned, and has been counted for the appropriate "I Made That" achievement. The bonus is not given for Dyes, PvP, Master, or Beast Tribe recipes, nor do they count towards those achievements.
This bonus experience is indicated on each recipe page. It doesn't scale nearly as fast as experience requirements per level, which makes it more useful at the lower levels. However, unlike gathering bonus experience, the bonus is the same regardless of your level when you first craft the item, so it's always worth getting these bonuses. A level 18 craft, for instance, yields 1/4 the bonus experience of a level 50 craft.
Experience Modifiers
When crafting an item, one or more modifiers may add to its base experience value. Base experience changes by level, so crafting an item of lower level than your class level will yield diminishing returns. Higher-level crafts yield much more base experience, though the quality increase bonus (see below) can offset some of these gains. Note that these bonuses are additive, not multiplicative. That is, two doublings (+100% bonuses) results in a tripling (+200% bonus), not a quadrupling.
Quality Increase Bonus
The largest part of the experience gained by crafting itself usually comes in the form of the quality bonus. This bonus is based on the amount of quality gained during crafting (not the amount of end quality). Thus, using high-quality materials to start with higher quality will actually limit the amount of bonus experience you can gain for a given craft. That said, the difference is not extreme. Taking any craft from 0 quality (no HQ materials) to full quality will yield a quality increase bonus of 300%. You start at half of maximum quality when using all HQ materials, and increasing quality enough to get to a 100% HQ rate will yield a quality increase bonus of 240%.
The quality increase bonus increases rapidly until it gets to the 240% "half quality" bonus, then levels off somewhat after that. It's important to note that this quality increase bonus is not related to actual high quality results (unlike gathering). You will gain this bonus even while crafting items that have no high-quality version, such as housing items. Thus, unless you are at the level cap or are crafting a recipe so low level as to be worth almost no experience, it always makes sense to increase quality as much as possible. Note that using quick synthesis will guarantee that you get no quality increase bonus, even if the created item is of high quality.
Rested Bonus
The rested bonus is a flat +50% modifier which accumulates while you are in a sanctuary, even logged out. It will automatically be used on any class that gains experience. While this bonus is easy to use, it is helpful to plan out how you're going to spend it. Rested bonus works as a percentage of level, not a flat amount, so it will be used up much more quickly at lower levels. Since these are typically the levels that are easiest to earn experience in anyway, it may help to try to use up your rested bonus on higher-level classes before crafting at lower levels.
Food Bonus
All food grants a +3% bonus to experience. While this is not a huge amount, you can get it very cheaply. There's really no reason not to use food when crafting. Further, food that enhances crafting attributes will allow you to gain more quality, thus increasing your quality increase bonus as well. The gains are often quite a bit higher than the base 3% bonus.
Company-issue Engineering Manuals
These manuals, purchased with Company Seals, give a +50% bonus to crafting experience for a duration of 3 hours. There is a limit to the bonus experience earned: 20,000 for a Company-issue Engineering Manual (1,440 seals), or 40,000 for a Company-issue Engineering Manual II (2,300 company seals). You shouldn't have a problem using up this bonus within the allotted time, though note that the timer still runs if you log out or do anything else. Also keep in mind that this bonus applies to base experience, so you'll need to do 40,000 or 80,000 experience worth of crafting, respectively, to use it up. You will generally gain much more than this base amount while actually crafting, so keep this in mind if you're working towards a particular experience goal.
If you will be away from your grand company headquarters while crafting, you can save a bit of time by buying a manual, immediately using it, then buying another (since they are unique). This is also why you might consider buying one of each manual even though the upgraded version is a more efficient use of company seals.
Tradecraft Leves
Tradecraft leves are the fastest way to gain rapid levels while crafting, at least until you run out of leve allowances. There are three types of tradecraft leve plates, each of which has its own advantages and disadvantages (described below). Unlike leves of other types, a given tradecraft leve is only available from a single NPC, but it is always available unless it is currently in progress. As a result you can freely craft the required items before actually starting the leve. All tradecraft leves grant a 100% bonus to experience (and gil) if you trade in high-quality items. It's always a good idea to try to obtain this bonus, even if it means trying a craft again. You're generally better off handing in HQ items for a next lower-tier leve than NQ results for the highest tier available.
The structure of the available tradecraft leves is much simpler than other leve types. There are six leves per class at every 5 levels from 1–45. Three are offered by the levemete in the same city-state as the tradecraft guild, and the other three will be at a level-appropriate camp. Each levemete has one leve for each of the three leve plates. (Below level 20, there are no Charity leves, so those are replaced by an extra Constancy leve.) The actual recipe level of the leve is not listed, but it does determine the reward and is generally in line with the level of the leve itself.
Constancy Leves
Constancy leves are the baseline crafting leves. They are handed in to NPCs in the same city-state or camp where the leve is given, and offer a large amount of experience. This experience scales nicely, offering almost 1/6 of a level at the top end and much more than that earlier on. To put it in other terms, a high-level Constancy leve is worth about ten times as much experience as the bonus experience for the recipe required. (At lower levels this ratio can drop as low as 3–4 times the bonus experience.)
While you can get more experience from other tradecraft leves, the main draw of Constancy leves is that you can repeatedly hand them in very quickly. Ingenuity leves require travel for each hand-in, and Charity leves require a lot more material. They're not optimal but they're often a good choice, depending on the items required for each leve available at a given level tier.
Ingenuity Leves
Ingenuity leves are virtually identical to Constancy leves, with two key differences. First, you must travel from the city-state where the leve is offered to the related camp for that tier (or vice-versa) to hand them in. Second, they offer a 50% greater experience reward than Constancy leves. It is entirely possible to go back and forth between city-state and camp Ingenuity leves, and this can be very effective if both are items you can dependably HQ. The only downside here is the time (and perhaps gil) spent traveling, but that time investment can be significant.
Charity Leves
Starting at level 20, Charity leves become available. These are much different than the other leve types. You'll need to craft three times as many items for them, and the experience reward is only 60% of Constancy leves. What gives? What makes Charity leves special is that they can be repeated, offering a maximum of three hand-ins (and thus 180% of a Constancy leve reward) per leve allowance. In terms of pure experience per leve allowance math, this makes them the "best" leves to use to gain experience. In practice it's not so clear.
The obvious downside to Charity leves is that you need to create 9 (!) items for each full leve hand-in. Fortunately they generally do not all have to be of the same quality (though you will get proportionally less of a bonus for handing in only 1 or 2 high-quality items). Both in terms of materials and crafting time, nine times the hand-ins is asking quite a lot. You do of course have the option not to do multiple hand-ins, but in doing so you're giving up part of your reward. (This can still be a good option if you just want to mark the leves as complete for achievement purposes.)
It helps to pay careful attention to the item requirements for Charity leves. For example, the leve Dancing with the Stars requires 3 Toothed Staghorn Staves per hand in. Assuming you craft the Staghorn Staff you'll be using to make the final item, that's four Antelope Horns per craft, or 36 total. Even if you buy the base item, that's still 18 Antelope Horns, which are hard to get in quantity and usually not very cheap on the market board. Compare this to the Fever Pitch leve, which requires one gathered and 6 gil worth of NPC-purchasable items per craft. The specifics make a very big difference.
Supply Missions
Once you join a grand company, you can do supply missions once per day. The company requests one item made by each of the eight Disciples of the Hand classes per day, all of which are level appropriate recipes. These are a great source of both experience and Company Seals. Basic supply missions will offer an experience reward similar to that granted by a Constancy leve for a recipe of that level. This is a significant chunk in and of itself, but it can be enhanced even further. Turning in high-quality items results in a 100% bonus to both experience and company seals. Some items will randomly be in "high demand," denoted with a star next to their name. These offer double the base experience and seals. Best of all, these bonus are multiplicative, yielding quadruple the normal bonus if you hand in a high-quality in-demand item. These are generally the fastest and best way to earn Disciple of the Land experience in the game, though they are of course limited to once per day per class.