Till the existing lawn.
Rake it smooth.
Lightly water the ground to prepare it to accept the sod.
Lay the sod strips out, butting them tightly up against each other. Cut them to fit with a sharp utility knife. Make sure seams are not right next to each other -- stagger them for better growth and less shrinking.
Water the new sod with a sprinkler for at least a half hour each day.
Wait a week to 10 days and fill the cracks between the sod strips with dirt.