Clear the lawn of any weeds or grasses using a glyphosate herbicide sprayed onto the lawn's surface. Wait for the grass and weeds to brown and then cut the ground cover into strips with a sod cutter. Pull the dead ground cover from the soil and discard.
Remove any stones from the lawn to keep the tiller blades from throwing them into the air and possibly causing injury.
Till the lawn with a powered tiller. Set the blades to a depth of 5 inches, then push the tiller across the lawn in overlapping rows, breaking up the soil as you go along until you've covered the entire area. Direct the soil into a gentle sloping pattern with a drop away from any structures of about a foot for every 100 feet of soil length.
Test the soil pH using a test kit. Send the test to a university extension in your area for analysis. Examine the results and add amendments to your soil as recommended by the tests to change the pH-balance to one that's suited for the sod type. Give the amendments a week to penetrate into the soil.
Spread any fertilizers needed for the sod over the soil and then run the tiller over the lawn again.
Rake the tilled soil level after the second tilling as final preparation for the sod laying. Make sure that the soil is about an inch lower than your desired grass level.