Compost acts as a soil conditioner and "improves the structure and texture of the soil enabling it to better retain nutrients, moisture and air for the betterment of plants," according to the University of Illinois Extension. Add compost to your backyard soil to improve it and prepare it for planting.
Set the depth gauge on your rototiller to 8 inches. Start the tiller and dig the tines into the soil area in your backyard where you want to add compost. Work in rows until soil is loose, overlapping rows by 4 to 6 inches.
Run a rake through the soil to find rocks and weeds. Remove the rocks and weeds by hand. Place weeds in a trash bag. Move rocks to other areas of your yard, such as around garden beds for edging or place in the bottom of pots to prevent water pooling to facilitate drainage.
Spread a 1-inch layer or more of compost on top of the tilled soil. Till the area again to mix in the compost. Work in rows again and overlap to thoroughly mix the compost into the soil in your backyard.