Use a belt sander to level the top and bottom of the tree stump. If the tree stump is extremely uneven, you may need to cut it straight across with a chain saw to reduce the amount of sanding.
Check the surface with a level several times as you sand.
Brush a clear wood polyurethane over the entire surface of the stump to prevent wood rot. Wait for the polyurethane to dry.
Set the stump in place in your home, and place the piece of glass on top of it. Position it so that it's centered.
The tree rings can be seen right through the glass to create a stylish tabletop.