Everyone knows about layering pieces of masking tape on the bed of your sewing machine to create an "edge" for your fabric to ride as you sew. I use a different method which is much more accurate and leaves NO residue. Buy a package of 3M Command Removable Mount Strips (Small size is what I use) and use that to mark the exact measurement of your seam allowance. You may need to trim the right-hand edge and a corner to make it fit your needs but it's easy to cut, pulls off cleanly with NO residue and is just the right thickness for your fabric to ride along. These strips are usually in the hardware section of your big-box stores but at this time of year you'll also find them in with Christmas items as they are great for hanging decorations that you want to remove after the holidays.