Joroas,
Here you go. These are quite straight forward. First of all here is the type of cork coaster that I use - a little cut up from earlier projects.

You trim the coaster to the size you want.

Than, using an exato blade, trim and bevel the edges.

Cut a hole out to represent the pit.

Now you need to cut the stakes for the interior. I have a package of slim dowels that I use but toothpicks, skewers, or matchsticks would work just as well. I carve them with an exacto and then cut them off and trim the bases.

Than you put down glue in the interior hole and place the carved "stakes" in with tweezers.

Prime with black paint.

Paint and glue on static grass.

Simple and straight forward.
LB