I make free-hand drawings of Celtic Knot work (I specify “free-hand” because people always ask where the stencil is). I saw this idea online a couple of years ago. I started drawing basic squares but my designs have gotten more intricate over the years. If you don't get it the first time keep trying! It takes awhile to perfect the steps involved, but eventually you'll be drawing them in your sleep.
Step 1: The Border
Step 2: Complete the Pattern
Step 3: Over Under
Over under over under over under over under over under. Do that about 500 times and you should end up with one giant knot. Take your time when doing this. Most likely, the first thing someone will do when they see this is follow the line, and if it doesn't go over under over under they will make sure to point it out to you.
Step 4: Finish
Go over the lines to make them thicker and straighter or even add more color.
Step 5: Get Creative
Instead of four sides do three or five, do a double border, make a frame, add circles and color, whatever you want. Have fun!