In reply to ROSP:
Carefully.
But seriously you can build an anchor on the top first, attach (with dynamic rope) the person who is going to catch the fall (avoid too much slack, you don't want to shock load it, but if you want to practice holding the fall you'll need a little slack so make sure the anchor is bomb-proof). Then the other person jumps (or climbs slowly) in to a crevasse and then you practice from that position whatever type of solution you like.
Post edited at 11:49