Do not use any leves on crafts under 20. And then, only if you can make the level 20 triple leve all HQ when you hand in - you might need to wait for 25 for some crafts.

Get ALL of your crafts to 15, at a bare minimum. You want the cross class skills. Stuff at that low level is fairly cheap, though if gil is an issue take up mining and botany to help provide materials.

You can basically do 1-15 or 1-20 just there at each craft's guild hall, just buying materials as you need them and making stuff in the crafting log. If you should, somehow, finish everything in the log without getting to level 15, then start making either staples that you will eat/drink (culinary, alchemy) or base level materials (ingots, leathers, lumber).