Slit seeders create an opening in the ground by slicing into it and creating slits, depositing seeds at the same time. It's a good way to get seeds in place to grow without having to worry about birds eating them or rain washing them away, but there are still a few things you have to do to get your lawn ready for the slit seeder.
Mow the lawn down the height where you normally keep it. Having a groomed lawn will help the machine run better because it won't be fighting thick, tall grass.
Aerate the lawn to loosen the soil and add air. This helps with any seeding project.
Rake the lawn to remove thatch, which is the buildup of dead grass. Thatch blocks out light and reduces air circulation, which can keep the seedlings from growing as well.
Fertilize based on the measurements recommended on the bag. The added nutrients will help the new seeds grow.
Water the lawn. Watering softens the soil and gives it moisture that helps the seeds germinate. Water it in the morning for several days in a row to make sure that it gets moist several inches down into the soil.