Stage 2. Using the soft pencil carefully draw the curved lines of the knot on to the grid. Work smoothly in manageable sections, and allow the lines to flow. Make sure the lines accurately touch the edges of the grid. Join the points with two longer vertical lines, to form the flat ends of the finished design.
Stage 3. Trace over the knot shape and transfer to a new sheet of paper, marking the outline as a dotted pencil line. Use the dotted line as a guide and draw around the outside using black felt-tip pen, keeping an equal distance all the way around. Measure the same distance inside the dotted pencil line and draw the window-shapes shown in the diagram. Take care to ensure that the lines are accurately spaced and the curves are smooth and flowing. When you gain experience you won't need to trace off the outline, but can simply rub out the grid and use the solid pencil line from Stage 2 as a guide.
Stage 4. Join together the black felt-tip pen outlines, making sure that the finished line always goes over and under itself to create te braided band of the Celtic knot. Use the diagram as a guide. Try to keep the band the same width throughout the pattern. Rub out the dotted or solid pencil line and add colour to your knotwork design.
This 4 stages tutorial is taken from the book Draw Your Own Celtic Desings, it was written by David James and illustrated by me and I think it's the easiest way to start to draw Celtic knotwork. The book was published by David & Charles in 2003, it's still available from any decent bookseller.