Double half hitch is what I think it's called. It's been a while. It seems like a granny, but it goes around the rope. I'll see if I can describe it:

- take the slack in your hand, as much as you want
- wrap it around the rope and through its self and tighten
- repeat

That should be it.

I'm an Eagle Scout, but like I said it's been a long while and I wasn't very good at knots