Select the pen tool and change the line width to at least 6 (the higher the number the thicker the trellis will be and dont forget if it's too thin the card may not hold the cut), use the grid lines as a guide and draw a diamond. I chose to make a diamond 1cm wide and 2cm long. The size at this point doesnt really matter as long as you have the ratio (how long to how thin) as you want it.
Save this image as a jpg, I called mine single diamond.
Select the flood fill tool again then click the white foreground colour in your materials swatch. The materials properties box will open now click the pattern tab. The first 2 of the thumbnails should be your saved diamond and the image you are working on. Choose the single diamond and click ok.