Clip into anchor with daisy or other sling with locker; two points of anchor are preferred. Take a length of rope up and tie an overhand-on-a-bight and clip this to a biner on your harness. Untie and thread through chains. Once you have sufficient rope through chains, unclip knot from biner, untie it, and continue threading the rope until you get halfway, or it touches down with sufficient extra. Sometimes, I'll thread the rope, tie another knot in the rope, clip that one off before unclipping the other, but that's being overly safe. Once you have sufficient rope to reach the deck, set up your rappel device and unclip from the anchor and descend.