To lay sod over grass, simply lay the new layer of sod on top of the old layer of grass, letting the grass die as the new layer starves it of sunlight and water. The nitrogen provided by the dead grass beneath the new layer of sod helps new grass grow.
The primary concern gardeners typically have about laying sod directly over grass is whether the new grass has time to take root in the primary soil layer within a few short weeks. Since sod offers few nutrients to growing grass, the grass must take root within the first week or two in order to gain access to the nutrients that keep it alive. Professional landscapers regularly lay sod over grass but often hesitate in situations in which the existing layer of grass is already flush with sidewalks, driveways and other borders, since the additional sod raises the new grass higher than the ground level.
Another way to lay sod over grass is by killing the existing grass first. Commercial herbicides can perform this task, but this process requires tilling of the soil after the grass dies. Additionally, it is recommended to test the soil to ascertain whether additional nutrients must be added.