If you can get some 1" gridded paper, that will help. Start your design on a specific horizontal line, then end it on the same line so the design will flow from one repeat to the next. The gridded paper will also help keep the design within your chosen width, but don't make it 'boxy'. Have a concave bit at the bottom edge and a convex bulge at the top so that you can nest your rows so they don't look like stripes. I guess just study some of the online designs for general pointers.