While rubylith is an option, you can absolutely use tape to create straight borders. I've done it this way for years. Look for 3M/Scotch safe-release masking tape for delicate surfaces. The current product is blue. There is an older version that was white that I like a little better (still have a stash), but the blue stuff works. Look for the lowest tack you can find. This will work very well on smooth papers like Platine, Fabriano Artistico HP, COT 320, etc. It's a little dicey on a textured paper like Rives BFK because it will pull up fibers if you press it down too hard. The white tape was the best for textured papers. I think there is a newer version of the white tape, but I can't find it with a quick Googling.