I have tried using a ruler on its edge to guide the brush to ink straight lines but i find it awkward and unwieldy and sometimes it slips which is a pain so my solution to this is to cut a length of 1" square timber to about 6 or 7 inches and flatten off one of its edges so that rests on the paper which will give you a left and a right side that clears the paper and gives you 2 edges to run the brush up and down.
