Prepare the top 2 to 3 inches of soil by loosening the soil with a tiller or with a hard rake. Remove any stones and other debris in the soil. Remove weeds by hand. Use the hard rake to break up big clumps of soil. Level the ground so that no low spots exist, or water will accumulate and wash out the grass seed.
When overseeding, mow the existing grass as close to the soil as possible. If there are bare spots, use a hard rake to break up the top 1/4 inch of soil. Remove any dead grass and debris. Level any areas with low spots with topsoil to prevent standing water.
Add starter fertilizer to the ground and seeds. If you are planting a new lawn, fertilize the entire area. If you are overseeding and covering bare spots, only fertilize the bare spots.