I learned how to do this in fourth grade. I never learned what it was called so I like to call it decorative squiggles. It is very simple to do, but also time consuming. You don't have to do it all in one sitting though. I also find it relaxing and it can give you time to just think.
Step 1: Start
Start with a paper or card that you wish to do the design on, and have a fine point pen ready. I am using a black Sharpie pen, you can use any color you wish or mix mach colors.
Step 2: Draw
First draw a shape. It does not have to be a squiggly shape it can be a rectangle or triangle.
Step 3: Draw Inside
Next draw in the inside of the shape following the edge but not touching all the way around and join.
Step 4: Finish the Shape
Continue following the edge and drawing smaller and smaller shapes inside until you can't anymore. You have finished one shape now.
Step 5: Continue
Continue by joining another shape to the completed one and make the smaller ones inside.
Step 6: Keep Going
Keep attaching shapes. This can be time consuming but it is worth it for how awesome it looks when it is done.
Step 7: The Edges
When you reach an edge do not complete the shape leave the edge of the paper open like so.
Step 8: Edges Fill
Continue filling it like normal but keep the edge open. This is not necessary but it looks nice.
Step 9: Finished
Once you have filled in the whole of what you where filling in you are done. Congratulations! Not many people have the patience for an activity like this.