You can purchase or rent an overseeder machine from a local hardware store. This machine has rotating blades that leave a groove in the ground, into which it deposits the grass seed. Confirm that the grass seed you purchase is right for the light conditions of the area (sunny vs. shady). You can then use this machine to plant the new seed in any thin spots of your lawn. 

The benefits of overseeding include:

• Fills in bare or thin areas damaged by diseases, heat, water deficit, insects, pets, and foot and equipment traffic. It also increases the density of existing turf.
• Reduce Risk of Pests & Disease: Overseeding will introduce new cultivars of grass seed that are more tolerant to pests and diseases.
• Reduces weeds: Creating a lush, dense turf, is the best defense against weeds. 
• Improves Appearance: Seed germination occurs in 7-10 days.
• Reduce Erosion: Grasses have fibrous, dense root systems that do an exceptional job of holding soil in place.   
•  Increases Thickness/Density: Turf becomes more durable, withstanding foot traffic and play, as well as holding up to natural weathering and environmental stress.
• Reduces Chemicals: Maintaining a healthy, robust turf can reduce the chemical applications needed.  

Before beginning the work, check the yard for any objects that can be thrown by the blades/machine. Rocks are a common hazard.

As an alternative to renting an overseeder, you can hire a professional like Weller Brothers Landscaping to take care of this. We have all of the equipment needed and use professional, high-quality seed mixes/blends.

After planting the grass seed, do nothing. You don’t want the seed to germinate until the spring because cold winter conditions could injure or possibly kill the young tender shoots  The winter snow will help with insulation and soil moisture once spring does arrive, reducing the need to water the area as frequently.

You can put down an erosion control blanket to help protect the area until the seed germinates in the spring. This will naturally biodegrade into the ground over time. 

Lastly, when mowing your new turf for the first couple of mowings, be sure to bag or rake the clippings. Removing the clippings prevents the clipping from collecting on the turf canopy and killing or injuring the young turf.