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A simple technique for accurate machine sewn seams involves a ruler and masking tape. Lower your machine's needle at the 1/4" mark. Leave the ruler in place and place several layers of tape one on top of the other butted up against the ruler's edge. This will leave a build up of tape that can be used to guide your pieces under the needle at just the right spot. Check the tape periodically to be sure that the mark has not shifted.
When I use tape to mark my 1/4 inch on the sewing machine I get the blue painters tape an use a razor blade and cut out about an inch wide piece and go through several layers (approx. 1/16 inch to 1/8 inch deep) of the tape and it will make a better vertical guide for your material to follow. The edge of the tape will be straighter than layering it yourself.