So you're making something that involves threading elastic or other material through a casing or tunnel. It's so tedious to sit there and push the elastic through little by little. Here's what I do.
1) Buy a bundle of elastic. Measure and mark your elastic at your desired length. Make an arrow so you know which side is the correct length.
2) Pin the elastic onto your fabric.
3) Sew the casing with the elastic inside, leave your usual opening. Be sure not to sew over the elastic!
4) Pull the excess elastic until you find your mark.
5) Stitch together, cut excess, and close!