1a. Draw a grid of dots, 6 by 6, all the same distance apart. The distance between them (A) is the measurement of 5 1/2 tesserae multiplied by 1.5. The same calculation as you use for the double guilloche border pattern. These dots represent the the single white centre tesserae.
1b. Once you've done this draw in dots between them within the square (B). I just do this by eye.
1c. The distance between the dots on a diagonal
should be 5 tesserae (C).
2. Start drawing in the pattern. Pick a line you can be absolutely sure is correct and repeat this around the pattern. Here I've picked the the top left centre tesserae. Continue this all the way around in a clockwise direction.
3. Now draw the line which forms the other side of the first line creating a strand. When you draw this in just start at the top of one of the curves and go down to the centre tesserae keeping a distance of 5 tesserae away from your first line. Look for any points which may help. These will be more apparent as you complete more of the pattern.
4. Now start inwards. Notice the under/over pattern.