Use a landscaping till to loosen up the top 3 inches of the soil.
Remove any rocks, roots or clods of soil that are larger than a half dollar. Break up soil clods if possible with the till or your hands, as you do not want the soil to be too fine. Rake over the soil right before applying the grass seed to ensure it is even and smoothed out, and make areas where excess water might collect as level as possible.
Prepare the soil a little differently if you are over-seeding an existing lawn. Mow the grass as short as possible by using the settings on your lawnmower. Remove any rocks, dirt clods, roots or dead grass from the lawn.
Use a till to loosen up the top 1/4 inch of the soil in any bare spots on the existing lawn. Smooth these areas over as much as possible, leveling them out so they are even with the rest of the lawn. Add some topsoil, if necessary, before leveling out the ground.
Spread the grass seed over the soil as evenly as possible, following the directions for the type of grass seeds and soil you are using.