A very simple and quick project! To attach the waistband elastic, I sewed the elastic together to make a band, then pinned in 4 places: CF, CB, and side seems. I attached the elastic by serging on wrong side of fabric, with blade up, then turned under the top stitched with twin needle.
Also made a point to attach a "tag" so the kid knows which side is back.
Graded out the waist to the size needed, and lengthened into pants.