If you have a sewing machine, you will have to stretch the elastic while you zigzag it.
If you want to hand sew it, you can use pins to divide the waistband into 4 sections, and use pins to divide elastic into 4 section. Then you can match the pins so as to pin waistband and elastic together.
When you pin them together, the waistband will gather, so you can sew one section at a time.
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