This, my friends, is a lark's head knot. I don't know why it's called that. I did research it a bit out of idle curiosity and from what I can see the knot itself has been around forever, referred to by names like "cow hitch" and "girth hitch" until some dude named Tom wrote a book in 1866 and dubbed it a lark's head. You call it what you want, but I'm sticking with Tom and calling it a lark's head knot since that's what I've always called it. This is one of those knots that can be made in several different ways, all resulting in a knot that looks like the picture above. Think of it like using different shortcuts on your computer that all get you to the same place.
I did this video first because this is the version of the knot that seems to give folks the most difficulty.
This version is useful for finishing off the end of piecess, but be sure to put whatever you want on the split ring before you start knotting (clasp or extender chain). I find it easiest to hold the piece in my hand since you need to be able to get to the underside of the split ring. I hold the split ring in my left hand to do the knotting from left to right and I try to get the cords I am not working with (the ones on the left side of the piece) out of the way as best I can.
Take the first knotting cord through the split ring from the top and through the loop in your cord and pull it snug for the first half of the knot. This first knot can be tricky since it's sometimes hard to know where your loop is. Just remember that the tail always goes behind - so if you're knotting from left to right, the tail of your cord needs to come out on the left side on that first half knot. It's hard to describe, but if you get stuck, try to remember that the tail goes behind and it will help. Hopefully.
To make the second half of the knot, use the same knotting cord, but this time go through the split ring from the underside. Then through the loop and tighten it up. If you're knotting from right to left, hold the ring in your right hand.
Whew! Now that you've got the hard version out of the way, let's try the easy one. This one is great for mounting new cords and is easy. Really easy.
See? I told you it was easy. But the two loose ends of your cord together and then find the resulting loop in the middle of your cord. Put the cord down through the split ring, then tuck the two loose ends through that loop and tighten. Bam! Done. Works around another cord instead of a ring, too, if you so desire.
And finally, another way to make the same knot. This time around another cord in the middle of a piece. Do it the same as around a split ring. Over the cord and around, though the loop. Tighten. Then under the cord and around, through the loop. Tighten. Ta da.
If you're having all kinds of fun and want to try another knot, check out the Double Half Hitch Tutorial.