I assume the OP has too many weeds, and/or thin grass areas that overseeding will help.
Just got this info from Dr. Green who did my place:
1. Cut your lawn just prior to aeration and seeding.
2. Don't water your lawn prior to aeration, it's easier for the aerator to take out soil plugs when it's dry.
3. After aeration, overseed the entire lawn, adding extra seeds in areas that are way too thin and you can see dirt.
4. Water daily for a couple of weeks for about 20 to 30 minutes for the seeds to germinate.
5. Do not cut your lawn for a couple of weeds after seeding.
6. Top dressing is not required unless you have large bare spots and want to seed the spots, then place top soil, seed, rake and compact. Water daily for a couple of weeks.
7. Note that each seed can make between 5 to 15 or so grass blades, there's a fallacy that one seed makes one grass blade.
8. When the seeds grow, thicker grass will suffocate the weeds and they will eventually die. A healthy lawn kills weeds.
Hope this helps.