If you are renting a stump grinder keep in mind these smaller model are better for surface grinds and taking out surface roots.
These machines are tough on the body without a doubt. Pretty basic principle, you are spinning a grinding wheel and taking a little bit off at a time so it is tedious.
Here is a video of the process.
Do not grind surface roots on living trees. Damage to living tree roots will introduce bacteria and other fungal diseases that can eventually kill the tree.
If you have roots showing it usually is because you are over watering. It is better to cover these roots with sand and let the grass grow above them.
Keep in mind you will create a lot of saw dust. This is excellent mulch to use in your garden and if you see a tree service nearby doing a stump grind, go ask them if you can have that mulch. They are usually glad to not have to deal with it and it is the best compost addititive I have ever used.