How to draw celtic knotwork from "Celtic Knotwork", by Iain Bain, Constable London, ISBN 0 09 469810 4
1) Draw the grid. u is the unit of measurment.
2) Mark off a margin, width n (1/2u) all around the pattern.
3) Spot the quarter-points, (1/2n or 1/4u) all around the pattern.
4) Draw in the diagonals.
5) Extend the diagonals that intersect the grid lines.
6) Draw the outter curves of the cords. Merge those at the ends into the boarder. Avoid kinks and reverse curves.
7) Draw the inner curves maintaining the cord thickness. Complete the corners with short straight lines.
8) Erase the lines to form a braid.