Add A Quarter Plus Ruler simplifies foundation piecing. Place the tapered edge of the ruler on the sewing line and fold back foundation paper. Remove the ruler and flip it around. Place the 1/4" lip against the folded edge of the paper and quickly trim the seam allowance with your rotary cutter.
If you've used a postcard to fold your paper against, you can still do that but the tapered edge of this ruler works as well or better. Watch the video below to see how it works.