An elastic casing is an easy way to put a gathered waistband on a skirt or pair of pants. Heather from The Sewing Loft shares two methods for sewing an elastic waistband. Both use a casing; one has you sew the casing before you insert the elastic, while the other has you attach the elastic and then sew the casing.
Click below for her tutorial:
How to Install an Elastic Waistband, by The Sewing Loft
[photo from The Sewing Loft]