1. Wrap the end of your floss around the tip of your finger, just enough to overlap it.
2. Roll your thumb and the floss up over the top of your finger.
3. Grab the roll and pull it downward towards the end of the floss.
4. There's your knot!
Since I've always done it this way, I don't really know if it's easy to get the hang of or not. So, try it out and let me know! Or, do you have another way? Here's another snappy technique I found. What do you think? :)