Rake up as much of the torn up bits of lawn as you can.
Sprinkle dirt over areas where the dethatching dug up the soil. This fills in the holes.
Look on the seed package for quantities needed for your lawn. Measure your lawn if necessary to get the size of the space that needs repairs. You can get the size by multiplying the length and width.
Place half of the required amount in a garden spreader.
Push the spreader over the yard in rows.
Fill the spreader with the other half of the seed.
Push the spreader in rows at a perpendicular angle to the first application.
Rake the soil gently to fluff it slightly and work the seeds into the soil.
Water the lawn for 15 minutes a day to germinate and grow the seed.
Cut back on the watering to once or twice a week as needed after the grass seedlings sprout up.