Drawing a Trefoil

Trefoils are pretty tough to draw. This isn't so much because of the difficulty in getting the circles drawn neatly enough, but rather it's because it's very hard to get everything positioned properly. Fortunately, I've developed three ways to work around this. The first method is to use a grid. In the example below, note the green lines; those are the same lines you're making in step 4 of the directions farther down the page.

An example of making a trefoil using a grid

Another method involves using some measurements to produce three pluses in a triangle. You can find that tutorial over here.

This last method is probably the best one for drawing it by hand:
Start by drawing a plus.
Place a mark one-sixth of the way up the top arm. If you need help finding the bottom sixth, just split the line into thirds and mark the midpoint of the bottom third.
Now place two more marks above the center of each horizontal arm at that same height.
Draw in two pluses: each plus should be the same size as the original and the end of their top arms should be at the marks above the original plus' horizontal arms.
Your trio of pluses are now in position to guide your creation of three interlocked circles. Add a mark to the ends of each arm to show you where the inner circles go. The closer to the center of the plus these marks are, the thicker the outline will be.
Add in your circles uses these pluses and marks to guide you.
Clean up the lines to finish your trefoil.
There is an alternate design for this symbol in which the central portion is removed. To make this design, just ignore that area when you're cleaning things up.