Aerate, then overseed. Aerating makes lots of holes for the seed to fall in and also puts those plugs of soil on top and the seeds can also grow in that.

1. Mow first on a short setting and bag the grass.
2. Aerate (make sure ground is not wet, but not dry either)
3. Then mow again and mulch those soil cores into the ground.
4. Then seed, then water.

Mowing short helps allow the seeds to get down to the soil where they need to be, not stuck in the grass. My toro has five settings, for overseeding I like to mow on the second lowest. Some people like to mow on the lowest setting but I think it's too stressfull for your existing lawn for an overseeding project.

When you aerate, go over the lawn twice in a criss cross pattern, a third time if you have time is even better. You need a bunch of these open holes to receive the seeds you'll sew. Finally, and most importantly, keep the seeds moist by watering twice a day. If you miss just one day, a good percentage of the seeds won't germinate. If temps are over 80, or it's windy and dry, you'll likely need to water three times daily.(seems unlikely in PA this time of year) You should see some germination after 5-10 days unless you're seeding K. Blue, that can take up to 3 weeks. You can go to watering once/day after the seedlings are about a week/ten days old. Good luck!

PS Very important..Buy the best seed available..it makes a big difference.