We are "lucky" enough to have quite a bit of moss growing in our backyard in a little "valley". If you don't have any in your yard, it grows in lots of public places, such as cracks in the sidewalk, cemeteries, etc. If you're really into this project and can't find moss, I hear you can order sections of it online...try a google search.
Place a layer of pebbles in the bottom of your clean jar. (This picture ended up being too full of rocks, so I ended up removing some later. I think a layer that covers the bottom in one layer of stones is sufficient.) You might also like to add some charcoal as an "odor eater", since this will be an enclosed environment.
Add a layer of loose topsoil, about 1/2" thick or more (depending on the height you prefer). Moisten the soil, unless you're like me and it has rained in Indiana for several days and the ground is already saturated.
Dig up a section of moss (or several pieces) from your moss patch. If you're using moss that is dry, moisten it too. Layer it on top of the soil and press down, making sure the moss is touching the soil.