First step is to open a blank image.
I opened my image 1200X100. I chose 100 in height because the tile I am going to use to flood fill with is 80X80, and I need some room to resize the canvas after all the work is done, so the pattern tiles properly. Create a new layer. Using black as the foreground color, select the line tool set to normal. Start a line at 100,0 and draw it down 10 pixels.