I've been drawing Celtic knotwork for quite a few years now so I thought I'd show you how you can get started doing your own. I've made a step-by-step guide to producing some very basic knots. The knotwork can be done using pencils, pen and ink or paint and brush, whatever your preference. Personally, I prefer a sharp (for accuracy) but soft pencil (2B - easy to erase) on marker paper as my drawings are coloured in with permanent markers. You'll also need a good ruler, some time to spare and lots of patience. Once you get to grips with the basics of knot construction there's limitless room for creativity and self-expression. So, don't give up, practice really does make perfect (though I've a long way to go yet!)
Using a pencil and ruler, make three equidistant dotted lines. Along the centre line mark off a line of equidistant dots.

Join these dots as illustrated below.

Next, draw a solid line around the dotted lines as shown. Be sure to keep these solid lines at an equal distance from the dotted lines. A certain amount of accuracy is important here, but don't worry too much, any mistakes can easily be corrected during the final stage.

The next step is the plaiting of the knot. This can seem tricky when drawing a very complicated knot but it's really just a question of following a single line and making sure that you mark the line following the same pattern, i.e. over, under, over, under, and so on...

Once you're certain everything is in order you can go over the lines of the knot with your pen, marker or brush, making sure to neaten any inaccuracies along the way. Watch especially that the lines of the knot link up to one another where they cross another line. Having completed the permanent lines it's now time to erase the pencil help lines. Do this carefully, particularly near the edges of your paper. Use the eraser in one direction to avoid any distastrous tearing or creasing of the paper - you won't want to ruin all your hard work!
The final stage is entirely left to your own creativity. You can make use of shadow, background colour, hatching, dotting, etc., to make your knot truly your own.
Make two dotted parallel lines with equidistant dots. The dots on the lower line should be halfway between the dots on the upper line.

Join the upper dots to the lower dots in a zig-zag pattern.

Join the zig-zags as illustrated.

Draw around the outside of the dotted lines making sure to keep an equal distance on either side of the dots.

Plait the lines as with the first knot and decorate to your own taste.

Those are the basics - the rest is up to your imagination. Have fun!