It's worth it to do your own research on what recipes are appropriate to grind on. I know you probably don't want to hear that (it's often easier to just be told what to do), but each player's material situation is different. Without over-explaining that, here are some simple steps to follow:

1. Choose a recipe that's 1-5 levels above your own. Some recipes are easier, some are harder, so try some out and choose one you can succeed at an appropriate amount of the times. Some recommend you have certain training manuals. If you have these, it goes much smoother and you can attempt a recipe much higher in levels than yours.

2. Choose a recipe that has few materials, or very cheap and easily obtainable materials. Also check how much you can sell it to the NPC for, since that offsets the price of the synth. If you've found a gem of a recipe that is really really cheap to make, then start on it earlier, even if you fail a lot, and keep grinding on it, even if you're getting 200 exp. You have to decide what your priorities are: low gil cost or low time cost.

3. Plan ahead. Some recipes make materials that you will be grinding on later when your level is higher. And some recipes make materials for grinding other crafting classes. There's a lot of synergy to be had, so take advantage of it.

4. Sorry for tooting my own horn here, but I wouldn't say it if I didn't believe it: there's no better tool for choosing your craft grinding path than mooglebox. Craft grinding was one of the primary reasons I made the website, and at that time I made the website for myself, without even dreaming thousands of people would be using it. Anyway, choose "Leatherworker" under the recipes section of the menu, and then in the "Level" column, type in ">10 and <30" (or something like that), to filter out irrelevant recipes. You can click on a row if the recipe interests you and it will mark the row pink so you can easily refer to it.