This is another pattern based on a Persian stucco border from the 13th or 14th century. It’s a bit tricky as the inner triangles are not connected to the grid lines (else it would just be a matter of drawing lots of parallel lines and colouring in alternate triangles). I found the trick was to draw the lines in step 4 quite close together in the middle. If you draw the triangles to be equilateral, it might be even easier!
Tomorrow, 10 of 14, a ‘Persian Flower Border’.