Barrell Knot

Start with a two ends pointing towards each other and overlap them ~ 1 foot.  Pass the Right hand rope (the one pointing away from you) over the other and back under

Cross the same end over itself making an X

Pass the end UNDER the X and pull tight

Flip the entire works around and do the same thing on the other side.  When you're done, you should have two sliding knots like these.

The finished knot.
Pull the outside ropes, and the two knots should slide together and the X's should mesh.   If they don't the knot is wrong and you should start over.