There are several ways to sew a 1/4" seam:
Take out a package of Dr. Scholl's™ Foot Padding. Using your ruler and an old blade in your cutter (or a new one - we are only making a short cut), cut a strip off the padding about 1/4" wide. Then segment that strip into pieces about 1/2" long. The only really important thing here is to cut it with your ruler so the side edge is really straight.
Go to your sewing machine with one of these little pieces. Take a short ruler that has 1/4" markings on it. Your 12" rotary ruler is perfect. Put the ruler under the needle and lower the needle onto the line that is 1/4" away from the right edge. Be sure the ruler is aligned on the bed of your machine so that it is parallel to the left edge. Remove the paper backing from your little padded strip. Put it down on the sewing machine just FORWARD of the feed dogs. Have it pushed up exactly against the edge of the ruler. Lift the needle. Remove the ruler.
Your wall gives you a physical and visual barrier that is precisely 1/4" away from the needle. If it extends over the bobbin case join, just use a razor blade to slice through the wall down to the seam in the face plate. This way you can open and close the bobbin case without moving the wall.
After many hours of sewing, the edge will become eroded. Replace with another piece.
When sewing with the padding in place, pin from the left toward the seam allowance. I keep the point of the pin just shy of the seam area so that I can sew without removing my pins left handed. If pins are put in from the right with the head hanging outside the fabric, every pin will catch on the padding or must be removed before getting to it.
No matter what method you use, you should perform the quarter inch test: If your new project involves fabrics that are markedly different from those you normally use, you should redo the test to account for the turn of the cloth. (see definition in Glossary).
Take three short strips of fabric cut 1 1/2" wide. Sew together. Finger press. Measure center strip. It should be EXACTLY 1". If it is not, adjust your method of measuring to correct (wider or narrower).