Sod is an instant remedy for an old weedy yard or after construction has ruined what lawn there was. In order for the sod to grow properly, it needs to have a good base for the roots to grow in. Just laying the sod over existing grass will probably cause it to fail after just a few weeks. Invest a little time and prepare the area covered by the old lawn for the new sod.
Spray the entire area with an application of 1- to 2-percent glyphosate in a garden sprayer at least two weeks before you plan to lay the new sod. Apply the herbicide, following all the safety precautions, in the morning after the dew has dried and at least two hours before any forecast of rain, wetting the surface thoroughly.
Cultivate the area to break up the top 2 or 3 inches of soil with a rototiller or tractor so that you get the soil finely tilled and crumbly.
Drop 2 or 3 inches of topsoil over areas with poor or sandy soil and rake smooth with a garden rake.
Soak the surface you want to cover in sod with at least an inch of water every day for two days before laying the sod, to settle the soil and to make sure the area is well-watered.
Lay the sod on a cloudy day if possible, butting the ends of each strip right up next to each other, tamping it down well to make good soil contact.
Water the newly laid sod twice a day for a couple of weeks until it resists your pulling it up. Reduce the watering to once a day in hot and dry areas or as needed in mild climates.