The Perfect Scant 1/4" Seam
1) Use a 1/4" presser foot, either with or without a guide. Check with your sewing machine dealer to see what the best foot for your machine is. You might be surprised that there are new choices out there!
2) Remember, if you do have a guide on the side of your 1/4" foot, it's just a visual guide. Don't jam the fabric up next to the guide, as that will probably make too large of a seam allowance.
3) Cut (5) 3” strips of scrap fabric (about 5" -6” long.) Sew them together lengthwise using a scant 1/4” seam. Press. Your piece should measure 13”. If it’s too narrow, make your seams a little bit skinnier. If it’s too wide, make your seams a little fatter. Do this exercise until you can get the right measurement without having to fuss too much when pressing.