I've attempted on more than one occasion to explain how I go about drawing Celtic knots, but finding it quite impossible to describe with words alone, I finally decided to put together this small tutorial with visual aid.
Most tutorials on drawing knots will emphasize use of a precise grid, but personally I don't feel that's always necessary. I start out by lightly sketching some guidelines for myself--as seen in the image above marked in red. I first divide my space into equal segments with vertical lines. Then I choose a basic shape for the knot segments and draw those in lightly as well. Here I'm using an oval shape, but any repeating shape (such as circles or rectangles), or repeating patterns of shapes (oval,rectangle, oval, rectangle, etc.) can be used.
Using the guidelines I just drew as boundaries for the general shape of the knot, I work out a pattern that fits within the shape. In this case, I'm using a relatively simple, symmetric 'S' shaped pattern. Asymmetrical patterns that alternate either horizontally or vertically are common as well.
The third step is the tedious part. Here, I draw in the double lines, adding in the portions of the lines that connect one knot to the next using the pattern lines I drew in step 2. Be sure to alternate in an over-under pattern wherever the lines intersect.
In the final step, I erase the guidelines (which may be unnecessary if you plan to digitally color your knots--you can simply paint over the guidelines) and I'm left with a nice, clean looking, evenly spaced knot.