I went over earlier the different ways of gathering raw materials in the game. What can be done with this stuff? Well, the easiest thing is to just sell it to someone else on the AH, your bazaar, or by spamming in some way. But what if you want to actually make something? How does that work, exactly?
There are 9 crafting guilds in the game and 8 of them work exactly the same way. The 9th, fishing, is mostly different but still participates in the guild point system discussed below. (This differentiates fishing from the other gathering professions.) The way you craft items in this game is you use an elemental crystal in your inventory to pop open a window with 8 slots. Hit enter on a slot to pick another item out of your inventory and add it to the crafting window. Put a bunch of stuff in the window and hit ok. If those specific items along with that specific crystal form a proper recipe and your skill level is within 9 of that recipe's level you'll try to make it. An animation will play and a couple seconds later you find out what happened. There are 5 potential outcomes from a given recipe: failure, normal success, high quality success, super high quality success, and super duper high quality success. On failure you lose the crystal and possibly some of the materials too. Each of the other 4 outcomes have fixed results based on the recipe and your chances for the higher quality successes is higher the higher your skill level is in relation to the recipe skill level.
For example, you could be trying to make apple pies. The skill cap for this recipe is 48, so you could try as soon as you were 39 but you'd get lots of failures at that point. Apple pies are food which give a 30 minute buff of 25 mp, -1 agi, 3 int. To make one, you'd use a fire crystal and then select a pie dough, a maple sugar, a cinnamon, a lizard egg, and a faerie apple from your inventory. Hit go and if you succeed you'd get 4 apple pies. Get lucky and you could get a high quality result and make 4 apple pie +1s. (Is that the proper pluralization of apple pie +1?) Apple pie +1 gives a 60 minute buff of 30 mp, 4 int, and 2 mp regen while resting. Get even luckier and make 8 apple pie +1s. Get the top tier result and make a full stack of 12 apple pie +1s.
Note you didn't need to learn how to make apple pies. Just putting the right combination of items together is good enough. You could search the game for hints on recipes or do massive trial and error. Or you could turn to the interweb and get full lists, like on these two sites.
There are all kinds of crazy theories about how to increase your odds of succeeding to make an item, or to increase your odds of getting a high quality result, or to increase your odds of leveling up. Matching the element of your crystal used with the element of the day or with the element associated with the cardinal direction you face! Or the phase of the moon, or the element of your equipped staff, or the weather going on! Nothing has ever been confirmed or denied officially. Here's a site that combines all the variables which they think matter into one spot to show where to face on what day when doing what recipe to get what result. Crazy!
Ok, so that's how to craft, but what about guilds? Why is fishing included here? Well, every craft has an artificial barrier that takes place every 10 levels. You can't skill up above level 10 without breaking that barrier and you do so at the appropriate guild. There's an NPC which asks for a specific item. Make (or buy) one of those items and trade it to the NPC to break the barrier and allow skilling up to the next 10. This is true for fishing all the way to 100 skill and for every other craft up to 60. For the other 8 crafts you can only earn a total of 40 skill levels beyond 60. So you could take one to 100, or 2 to 80, but you can't have 2 at 100. Some recipes actually use multiple crafting skills so this restriction actually limits who can make certain things.
Guilds will also provide you with a temporary skill boost if you ask for it. +1 skill for free or +3 for a fee.
Finally, you can sign up to be a premium member of one guild. Every earth day an NPC in that guild will ask for an item and will award you guild points for trading them that item. There's a cap on how many points you can earn in this way each day and pretty awesome rewards when you gather enough of them. Gear and furniture that increases your skill, or unlock the ability to make certain things, or let you sign your work. (I guess I lied when I said you could make everything without learning the recipes since you can't make sushi, for example, without buying the sushi kit thing from the cooking guild.) You can switch guilds at any time but you lose all unspent points when you do.