In a perfect world, you'd only seed in the late summer (August or September). In the real world, seed if you have a bare dirt spot...it is always better to seed it right away than to leave it bare.
If it’s bare dirt you’re job is pretty easy. Use a garden rake or some tool to loosen the soil, (we love the Garden Weasel) spread the seed at the recommended rate and lightly work the seed into the soil. To help it even more cover it with a little compost and work it all in together.
If you have a lot of thatch or dead grass on top of the soil you’ll either need to really loosen it up to expose some real dirt to seed into, or you will want to spread a light layer of real topsoil (not peat moss). Spread the seed on the fresh topsoil or freshly prepared ground and lightly work it into the top ¼ inch of soil. Cover it with a light layer of compost or pelletized newspaper mulch (the green pellets).
If you’re bringing in a fair amount of topsoil you probably will be better off seeding in the late summer or early fall as opposed to the spring or summer (Topsoil carries a lot of weed seeds that will sprout better in the spring and summer). Keep the area wet for 3-4 weeks to get the grass to sprout and water it a couple times per week until it is fully established.