I really don't understand crafting.

[Felis]

If you are in a grand company (around lv20 storyquest, I think) you can also craft stuff for the company. You can find the list what they need in the Timer window UI or with speaking with one in the GC headquarter.
They give good exp, GC seals and points for your GC achievements.
HQ items give you double rewards. The one with a yellow star give you also double reward. If you do one with a star as HQ it give you 4* the rewards (double + double).
But you can only craft 1 item each crafting class, then it vanish from the list after you give it to your GC.
After a specific time (7:00pm GMT in europe) the list get reseted and you have a new list.