Just a quick trick I use when inserting elastic. I measure the elastic per the instructions and mark with washable marker leaving 1" tails on either side. The length is then 2" longer than needed. Having the 'tails" makes it so much easier to sew the ends securely at the casing edge. Simply line up the mark with the edge of the casing sew and then trim. The "tail" gives you just enough to hang on to the elastic and easily position everything under your needle.