There are about as many ways of attaching elastic to a waistband as there are types of elastic and types of waistbands. However, two approaches are the best for novice sewers. The first involves attaching the casing after the elastic is in it, helping it to keep from rolling. The second is a simpler method that gives equally good results.
Cut the elastic between 2.5 and 10 cm (1 and 4 inches) shorter than your waist measurement. You need to have it smaller so that it fits snugly and won't be uncomfortable.
Cut a casing three times wider than the elastic, or at least enough to give plenty of room to fold over with the right sides together and stitch the bottom closed. Leave the ends open.
Turn the casing right side out.
Hook a safety pin to the end of the elastic and slide it through the casing.
Push material on the end of the safety pin and then, from the outside, grab the end material and end of the safety pin and push the material down over the attached elastic. Do this until you have reached the other end of the skirt.
Secure the elastic in place 1.5 cm (5/8 inch) from the end, tacking in place on one side of fabric of the casing.
Sew the casing onto the skirt, stretching it as you go.
Cut enough material so that the waistband will fit the length of the skirt plus 1.5 cm (5/8 inch) for each end to allow for a seam allowance.
Double the width of the elastic and add 3.7 cm (1 1/2 inches) for seam allowance.
Attach one edge of the band you just cut to the inside edge of the skirt. The right side of the band should be placed on to the wrong side (the inside) of the skirt.
Cut elastic about 2.5 cm (1 inch) shorter than the length you want.
Pin the elastic in place after folding one end of the seam allowance over it. Tack it down. Then do the same for the other end.
Fold the cloth of the band over the elastic toward the outside of the skirt. Fold under seam allowance and pin the band into place. This encases the elastic in the band.
Close the end of the band by stitching across one end. Continue to top stitch the band onto the skirt, but be careful not to stitch on the elastic. Stretch the material as you stitch. Close the casing by stitching closed the other end.
To save a step, if you aren't going to have multiple pieces of elastic in one casing, pin the elastic to one end of the casing before turning it inside out. As you come to the end of the task, stretch and pin the elastic to the other end. For a little variety, you can make a wider waistband and sew multiple casings by just making a few extra lines of stitches next to each other. Do not stitch the separate lines all the way to the end. Each separate compartment can be a casing for a separate piece of elastic, or the two outside ones can be casings. If you are concerned that you may lose the far end of the elastic in the casing, as you approach finishing pushing it through, tack the end of the elastic 1.5 cm (5/8 inch) from the beginning point to one side of the fabric. Push each end of the casing inward and top stitch down.