Summarize the following:
Use elastic that’s between 1/2 inch (1.27 cm) and 1 inch (2.54 cm) wide. It should be about 4 inches (10 cm) long, or 1 inch (2.5 cm) more if your hair is extra thick. Your final piece of fabric should be about 8 inches (20.32 cm) long and 4 inches (10 cm) wide if you use 4 inches (10 cm) of elastic. Add 2 inches (5.1 cm) onto the length if you added 1 inch (2.5 cm) to the elastic length. There's no need to adjust the width of the fabric. Fold the longest side of a rectangular piece of fabric at about 4 inches (10 cm) from the edge in order to cut along the fold. Remember to always cut a little bit extra fabric just in case you need more to sew. Feel free to cut past your initial measurements. Oftentimes you can take fabric in, but you can’t take it out once it’s cut.

Summary:
Measure and cut the elastic. Measure the fabric. Cut along the edge of the fold with sharp scissors.