A monkey fist knot is basically three (or more) turns of rope, three more turns around the first three, and a final set of three more turns locking them all together.
Two good sites (with pictures) of how to make a monkey first knot are:http://animatedknots.com/monkeysfist/
Start about 10 feet from the end of the rope -- this'll give you about 6' for forming the knot + 3' for the end braid + 1' for tightening...
The initial knot has to be pretty loose, otherwise you'll never be able to feed the last three loops through the knot.
To tighten the knot, start with the rope at one end of the knot. Almost as if you were trying to untie the knot, push/pull the rope into the knot, creating a loop. Following the path of the rope, push/pull the loop through the entire knot, pulling the loops tighter as you go.
Cut/trim the rope as needed to end up with a monkey-fist knot in the middle of two 3' ends of rope.