However, there are times when it is the only way which is guaranteed to work. I'm piecing these strips of 24" long half square triangles. My favourite method for half square triangles is to draw a grid of the finished size plus 7/8", stiching 1/4" either side of the diagonal and then cutting it all apart. This is fine for most purposes and is accurate. This time I need more than accurate as the length is critical so I'm foundation piecing.
I use freezer paper as it adds to the accuracy and I starch the fabric within an inch of it's life.
Here's how it works for me.
I stitch on the line.
I then move it to a cutting board, fold the freezer paper back and cut a 1/4" seam with a ruler and rotary cutter. (If you cut it back with scissors there is a chance one of the fabrics will show through the other, but this is fine with medium and dark colours)
I fold the freezer paper back in position and flip over the piece I've just sewn on. Carefully I press this seam open.
I flip it back over and press the freezer paper on to the new piece of fabric. I love these little stick irons for this. I used to use a travel iron before I bought one.
When all the pieces are pieced I use the edge of the freezer paper and cut my 1/4" seam. Notice I haven't drawn my foundation with this seam included.
And here it is.
I won't remove the freezer paper until this unit is enclosed by other fabrics all the way round, or the top is finished. I use very fine thread and small stitches. Before I pull it out, I run the back of the unpicker along the seams, this pressure breaks the paper between the stitches.