How to Tie a surgeon's loop
This is a video shows you how to tie a surgeon's loop. You start with a large loop, and treat it as one. Then just tie a normal overhand knot, but instead after that you pull it through once more as shown in the video. Then pull both ends to tie the knot up. This is a very strong knot so it's a very good loop to know. Their website also has much more information on how to tie this and other knots.