There are a lot of different things that can be combined around the different areas of the game, and you will have to unlock this achievement eventually because some things need to be combined to progress.
The most important things to remember are:
Gunpowder + Gunpowder = Handgun ammo
Any Herb + Any Herb = Mixed Herbs
You'll probably be doing those two combos a lot. Plus, they're easy to do at the beginning to unlock this quickly.