If you only need a small piece for a straight edge, you don't need to cut on the bias, which can save you some fabric. To start, you'll need cording; I like 5/32" cording, or even smaller. You'll also need a zipper foot or a piping foot. Wonder tape will help, but isn't necessary.
- Cut a strip of fabric the length you'll need by 1.5", plus a matching length of cording
- Place a strip of wonder tape along wrong side of one long edge
- Place cording along wrong side, in centre of strip
- Fold in half, matching long edges, and with cording in between, pin or clip
- Using zipper foot, baste cording in place, snug, but not too tight.
Alicia from Swoon made a very thorough and great tutorial here.