There are several ways to rig a top rope belay. Following are the common elements for all top rope belays:
The rappeller rappels down Rope #1 as normal.
Rope #2 is tied to the rappeller's harness and runs up toward the anchor.
Once Rope #2 gets to the top anchor, there are a couple of options:
Rope #2 can be fed through a belay device that is attached to the anchor.
Rope #2 can be fed through the anchor then through a rappel device that is attached to the belayer's harness (either at the top or bottom of the cliff, it doesn't matter).
Caution: You may be tempted to run Rope #2 from the rappeller directly up to the belayer at the top of the cliff. However, if the rappeller loses control, he would likely pull the belayer off the top of the cliff.