Just take the outline shape of a baby's hand and trace it on paper, cut that out and use it as a pattern, sewing with right sides together sew the opposite edge from the thumb together. attach elastic about a 1/4" from wrist seam while stretching it. I use a zig zag stitch for somethign like this. leave a tail of elastic trailing over the thumb edge of the mittens. Don't forget to press a bit of fabric down to finish the upper edge above the elastic. Close up the rest of the seam.