I routinely make flashes as follows. Start with 1/4 yd. of fabric. I then cut 4 strips parallel to the selvedge which are 2.5 in. wide each. A roller cutter/cutting board and straight edge work well for this. I then press these over a cardboard pattern which is 1.4 in. wide. This helps to keep the edges straight with 0.5 in. folded under on each side. Take care to make sure that the correct side of the fabric is used here and that it is oriented the same way each time. I then stitch down the hem along the long edges as close to the outside edge as possible using the sewing machine. I use a thread that is the same colour as the fabric. To keep things from fraying, I then cut away most of the excess fabric on the reverse side with pinking shears. I then double fold over and stitch down one end of the strip to make a sleeve to hold a 3/4 in. wide elastic band. I then trim the 4 strips to the desired length. To make the notch in the end, I mark out a 90 deg. angle with tailors chalk using a drafting triangle as a guide. After cutting the notch, some anti-fray compound can be applied to the cut edge.

Rick