To earn Expert Herbalist in Ghost Recon Breakpoint, you must collect 50 natural resources. This should come easily enough in a game so massive and long, but to speed up the process, there is a skill tree ability you'll want to unlock early.
Under the Recon section of the skills menu, the ability named Resource Detection will put all natural resources on your mini-map. From there you just walk over them as you always do, only now you can spot them much more quickly.
If you don't want to spend a skill point on that ability, try searching the shores along different lakes and rivers. There tends to be a lot of resources there.