Remove all weeds from the lawn. Weeds steal nutrients and water from existing grass and germinating grass seed. The weeds and roots may all be dug by hand. Broad-leaf weeds may be sprayed with the appropriate broad-leaf herbicide approved for use on lawns following the manufacturer's instructions. Grassy weeds and broad-leaf weeds may be treated with a glyphosate, broad-spectrum herbicide. Read the manufacturer's instructions regarding the amount of time you must wait before re-seeding the lawn.
Remove thatch (the layer of dead grass and roots at the base of the grass) from the lawn if it is thicker than 2 inches. Use a garden rake --- a long process --- or power rake, according to the manufacturer's instructions.
Fertilize the lawn with 1/2 pound of nitrogen per 1000 square feet.
Water the lawn to moisten the top 6 to 8 inches of the soil immediately prior to slit-seeding.