The possibilities for creating Celtic knots by hand are endless, their level of complication determined by the number of strands used and how you decide to connect the ends.
You can create varying difficulties and intricacies of Celtic knotwork. See the bibliography for other ideas.
Draw four straight, parallel lines of roughly equal length with a highlighter or wide, straight-edged, light-colored marker. Use a ruler or graph paper to produce more exact lines, if desired, and for the most precise control, use graphic or drawing software.
Draw four equal lines perpendicular to the first set of lines, creating a crosshatch pattern.
Outline the lines with a pencil or pen to complete one side at a time. The first outline should cross the perpendicular line, and the next outline should stop right before the next perpendicular line and resume directly after and continue in that manner. The outline will create the illusion of a weave pattern, and the lines will look like strands in the weave. The strands follow an over-under-over design, making it appear that one strand will go over the first perpendicular strand, then under the next.
Outline the under/over pattern for the next strand. This time the pattern will be opposite: outline to the first perpendicular strand, then resume on the other side. Continue this outline over the next perpendicular strand, and so on.
Outline the final two perpendicular strands, over/under then under/over.
Turn the weave pattern sideways so you see the perpendicular, unlined strands. The pattern will once again be over/under, and the pattern of the strands will run opposite to the original strands. Use the gaps left by the completed original strands as a guide to what you need to draw.
Connect the outside lines on a pair of strands and then connect the inside lines of the same strands with simple curves. A compass or protractor can be used to make the curves clean. Do the same for each pair of strands in the weave. Experiment with different numbers of strands and ways to connect the pairs of strands to produce different knots.
Odd numbers of lines will leave loose ends. You will need to find something to cap off those ends, such as a triangle, curve, or for a fun effect, a snake or dragon's head.
Davis, Courtney"Knotwork and Spirals : A Celtic Art Workbook", 1999
Making Celtic Knots by Jo Edkins
Meehan, Aidan: "Celtic Design: A Beginner's Manual", 1991
Two Ways to Draw Celtic Knotwork by Daniel L. Isdell