Established lawns can be over seeded to serve a few different purposes. Overseeding with the same seed as is presently growing can be done to thicken up a thin or damaged turf. Overseeding with a complementary grass seed can help to create a year round or longer season lawn by mixing cool- and warm-season grasses. Overseeding can readily be done as a DIY project in a few hours or a day if your lawn is very large.
Aerate or power rake your lawn to thin and lift excess thatch. This will boost the health of the base lawn grass and also allow the overseeded grass and applied nutrients to more easily saturate the soil. Make two passes over the entire expanse of soil raking up and discarding the gathered debris when finished.
Throw down at least a half-inch to 1 1/2-inch layer of good quality of compost or bagged garden soil mixed with well-aged manure. Cover the entire lawn area that you will be overseeding in an even layer to serve as a bed for the grass seed.
Sprinkle over a granular starter fertilizer for lawns. Use a mechanical spreader or simply hand cast the fertilizer over the entire area. Use the amount of fertilizer recommended for the square footage you are covering. Do not over fertilize as it can burn the existing grass if used in excess.
Sow grass seed by hand casting or using a mechanical spreader. Lay down the grass seed at the coverage rate recommended on the label of the grass seed package. Depending on the needs of your grass seed, leave the seed exposed to the sun or cover the seed with a very thin layer of more compost or bagged soil and aged manure.
Water the new seeds in deeply after sowing and keep the soil and seed very moist at all times through the germination and establishment phases. Depending on the climate, this will likely require daily waterings and sometimes twice daily waterings in warm or dry climates.