First step is don't loosen the soil. Grass seed does just fine with firm soil.

Second step is don't apply more topsoil and don't mix the seed in with the soil.

The main thing is to determine why the grass isn't growing. Is the thin spot on the north side of a building or the south side? Was it worn away and didn't come back? Is the thin spot also a shady spot?

Do you know what kind of grass you have?

Do you want a showplace lawn or just a lawn that keeps the mud off your shoes?

Will there be pets or children playing on or in the grass?

Grass care is simple if you allow it to be. 90% of it is to deeply and infrequently when you water, mulch mow it high almost all the time, and fertilize it 3x per year (Memorial Day, Labor Day, and around Thanksgiving). Preemergent herbicide in April is good and again in the fall. If you need to reseed, fall is the best time of year with January coming in at a distant second best. Still further behind is the spring with summer being the worst time to seed northern grass types.