In reply to PJay: I think you are getting unecessarily confused. Here is what you would do:
1. Ariive at crag;
2. Select appropriate route, with suitable belays at both top and bottom;
3. Go to top of crag; rig belays with all usual caveats, especially rub points and belay krab over the lip of the crag;
4. Uncoil rope; clip middle of rope to belay krab. Throw both ends to the bottom of the crag. Descend to bottom of crag;
5. Pull rope through until one end of rope has only about 2 metres on the ground. This is the live end of the rope;
6. Construct belay on the ground, using a combination of slings and the other end of the rope. The belay must be rigged for an upwards pull. I often tie the rope into belays using a bowline on the bight to equalise the anchors;
7. Tie a knot up the dead rope, into which you will clip the belayer; OR tie a knot which can then be used as a direct belay. If the ground below the crag is steep, I might tie a bowline on a bight here too, with one loop short, and one a bit longer - the belayer clips into the short loop and attaches the belay device to the longer loop.
So, really nothing complicated...a 20 metre climb would use up 40 metres of rope; add 2 metres for tying in and there is still 18 metres of rope lying there on the ground. How big a bottom belay were you thinking of?