First I cleaned the entire interior of the jar with some bleach since you want to deter mold as much as possible. Then I put a layer of small gravel rocks, about an inch high, on the bottom of the jar to produce drainage. I got the moss from my backyard. I dug it up using a trowel and got the dirt from the same place as the moss. Something about the acidity in the dirt.
Then you water it but not too much. And that's pretty much it, or so says the internet. I really hope it goes well, because I'd love to do a Venus fly trap terrarium as well. When I lived in an apartment by myself about 3 years ago I had several Venus fly traps but I could never keep them alive because I didn't know the best way to regulate their moisture. I tried a lot of things and most of them failed. Now I'm excited at the prospect of such compact and easy container gardening.
Look at how beautiful that moss looks with all those little leaves. So neat.