The Completion Knot can be used with both the Round and Square Knots. It seems a little complicated so don't get discouraged if it takes you a couple tries to get it right.
Step One- Leaving yourself at least a couple of inches of cord, tie one more knot, but do NOT pull this one tight. Leave it loose so you can thread the ends through the knot in the next steps.
Step Two- Take any end and wrap it around the end next to it, then up and through the hole in the center of the knot.
Step Three- Do the same all the way around with each end.
Step Four- You can see in this picture where the end goes in and comes out.
Step Five- Once you have all four ends threaded around and through the center, pull them tight until it looks like this.
Then all you have to do is trim the ends, add some beads, singe the cords, whatever tickles your fancy!