Anatomy of a Trefoil

You've looked and looked at these Celtic knots wondering, "How'd they do that?"  Here's a basic trefoil in 5 steps.  Once you've mastered this one, try your hand at more complicated designs.

1).  Begin by lightly drawing  the pathway--you're going to erase this later.
2). Draw the inner borders.
3). Draw the outside border.
4). Draw the "overs" and "unders"
5). Erase the pathway.