Clay soil is a difficult medium to work with because it is so tightly compacted. It is heavy and thick because it's comprised of many small particles that stick together and cling to wetness. There are hardly any air pockets under the ground, so it's difficult for plants of any kind to push through and develop strong roots. The key to working with clay soil is to amend it to make it more fertile and ready for plants.
Remove rocks and branches from the planting area. Wedge a shovel under large rocks, if necessary.
Dig up old grass and weeds with a gas-powered sod cutter. You can rent them from a nursery or planting center. Removing the grass and its weeds will enable you to plant something else in that area.
Loosen the clay soil to add air pockets underneath the surface. This will allow the roots of whatever you are planting to spread and develop. Dig the rototiller into the top 6 inches of clay.
Pour topsoil into low spots and dips to make them level with the surrounding ground. Rake the soil until it is smooth.
Make the clay soil more fertile by adding compost and other organic materials. According to Gardening Know How, add 3 to 4 inches of well-rotted manure, compost and leaf mold on top of the clay. Till it to combine into the top 4 to 6 inches of earth.
Figure out the soil pH by using a test kit from a planting center. Most plants need a relatively neutral pH to grow properly. If the pH of the clay is between 6.0 and 7.5, you won't need to make any changes.
Adjust the soil if it does not match the necessary pH of the plants you plan on growing. The Garden Helper suggests increasing the pH of acidic soil by adding hydrated lime. Spread 12 oz. of lime per square yard to boost pH by 1.0 point. Reduce the pH of alkaline clay soil by adding 3.6 oz. of ground rock sulfur per square yard.
Boost the fertility of the amended clay soil by using a starter fertilizer prior to planting. The package will tell you how much to use and what setting to use on the broadcast spreader.
Combine the fertilizer and pH soil amendment, if you added some, into the top of the clay.