Typical procedure for overseeding a lawn can be laborious. First, you need to use weed-killing products, rake debris, mow the lawn, aerate the soil, then spread the seeds, constantly water etc. Here we will go over an easier, and importantly cheaper, way to get optimal results while not getting overwhelmed in the process. You can follow this in early spring or fall.
Select a brand of lawn seed that is manufactured locally. This is very important as the seeds will be suitable for your weather conditions. Most home depot and garden stores should carry variety of local brands.
Cut your lawn (with lawn mower bag attached) to be about 3-4", so that excess grass or weed doesn't prevent the seed from reaching the soil.
Spread seeds. Based on the condition and size of your lawn you can decide whether to invest in a rotary spreader or dropper. You can simply spread using hands, the old fashioned way. This can actually be more effective as you can increase or decrease the seed concentration as needed. I.e. if you come across a bare spot, drop more seeds. If not, just spread mildly.
Cut your lawn again, this time use the maximum setting in your lawn mower and also mulch the lawn, i.e. don't use the bag type. This step is useful for several reasons. It shakes up lose seeds and increases the chance of soil contact. It also covers the seeds with grass mulch so that they are protected and have a better chance of growing.
If you time your overseeding to coincide with 2 weeks of rain and sun, typical spring weather, you don't need to do anything more in terms of watering the lawn etc. Come summer you will have a beautiful lawn.