What's a great easy to understand guide to Leatherworker?

Not trying to sound negative, but if you get a crafting class to level 60 and want to actually try to do some high level crafts, having every single crafting class to at least level 15 is a must. To make other crafts even possible, having many classes, if not all of them, AT level 50 is a must. Unfortunately there is no leveling up just one crafting class and that be the end of it (Unless you are okay with LITERALLY never getting high quality crafts)

But to answer some of your questions, yes the mentor system could help if you find someone willing to explain a lot of the crafting process. I'm not sure how many guides there are when it comes to leveling up, but I'll tell you what I did to get my crafting classes to lvl 50, which personally worked well for me.

1-15 buy materials from the guild masters and craft as many new crafts as you can for bonus exp and just craft literally anything to get to 15. The materials hardly cost anything which is why this part is easy.

15-30 I use this crafting levequest guide to find levequests that not only give a lot of experience, but also are cheap to buy from vendors. For example, if quest #1 gives 10k exp and the item costs 1k to buy, I would much rather pick that quest over quest #2, which gives 8k exp and the item costs 2k to buy from a vendor.

30-50 This is when I start to actually craft HQ (high quality) items to turn in for DOUBLE experience. I use the exact same levequest website to determine the quest that gives the most experience every 5th level.

How to HQ items? That's a whole 'nother story which requires all the crafting classes to be at least lvl 15 (which actually isn't that hard to get). There's probably guides or youtube videos out there that show how to HQ 2.X crafts, but it's a bit too much to explain right now.

/r/ffxiv Thread