Fog of War/Hidden enemies

Hi, I've just signed up and am getting my group to play on roll20, and I've been checking the help wiki and I can't quite find my answer yet. How do I "hide" enemies? Or is this not even really a thing? Basically I've laid out a map with some trees, and put some enemy tokens waiting in those trees. Do I just not reveal their spots with the fog of war? Or is there a simpler way to make these enemies hidden to my PCs? Thanks for any help!

you could leave the enemy tokens on the gm layer and then when it is appropriate you can changed their layer to token layer. Everything on the GM layer is invisible to the players.

Ah, that is exactly the answer I was looking for, thank you so much,