Elastic is a useful addition to many garments, such as sweatpants, shorts and linen pants. What's not so useful, however, is the tendency of elastic to twist up inside the waistband, restricting expansion and potentially causing some discomfort. You can prevent this from happening by following the steps below, whether you're making a new pair of pants or altering an existing pair.
Select a thread color that matches or complements the color of your garment.
Make a small incision inside the waistband using your craft knife. It does not matter where the incision is made. Pull out the elastic.
Slide the upper portion of the elastic waistband sheath under the arm of your sewing machine. Flip the foot down to secure the fabric.
Run the waistband through the machine approximately one to two millimeters below the upper edge of the waistband itself. Continue around the entire circumference of the pant.
Finish the stitch. Snip the thread to free the garment from the machine.
Feed the elastic back through the casing, using a straightened coat hanger or safety pin to guide you.
Close the small incision you made earlier by hand-stitching with a matching thread and a hand-sewing needle.
You can make vertical stitches through the waistband and elastic to lock the material in place. This is called "stitching in the ditch." Be aware that this procedure reduces the stretchy properties of the elastic.