Use a carabiner to attach the loop to one of the anchors. This loop is called a safety loop, and it should be slack.
Have everyone but the last person rappel down.
The last person should verify that the knot did not get jammed or pull through the rapide.
Attach your rappel device to the rappelling strand of the rope.
Unclip the safety loop from the anchor.
If you're comfortable that the knot block will not pull through the rapide, you can put the carabiner away. Otherwise, clip the carabiner to the pull strand of the rope. Be advised that this makes the retrieval slightly more difficult.
Attach the pull cord to the end of the rappelling rope (on the non-rappelling side).
Toss the pull cord to the bottom.
As you rappel, verify that the pull cord reaches all the way to the ground.
At the bottom, untie the stopper knot then pull on the pull cord to retrieve your rope.