Here is all that you need to know: when the snow melts during a thaw and you can see bare spots or thin areas of lawn, simply sprinkle some grass seed fairly heavily right over the soil or thin grass areas, and that’s that. Over the next month or two the freezing and thawing of the soil along with the water penetration from snow and rain will work the seed into the soil. It will be deep enough to keep it moist and protected. This seed should sprout by mid-spring. It may not be quite as effective as conventional seeding, but it is fast, easy and worth a try, especially if you have some leftover seed sitting around that should be used before it is too old to germinate.
Winter seeding should not be done on lawns with a thick thatch layer since that would prevent the seed from reaching the soil. Also, there might not be any point to winter-seed low areas or other situations where you are going to need a lot of soil to build it up. But if you have a dog that is muddying up the yard then winter seeding may be the only way to seed the lawn without blocking it off from the dog in the spring.