Attaching knicker elastic can be a little tricky the first time so I've added some step by step instructions to help guide you through the process. I highly recommend buying a little extra and practicing first.
1. Measure along the edge of the opening and reduce by 1/3, cut the elastic to this length.
2. With the right side of fabric facing up and the straight edge of the elastic aligned with the edge of the fabric, attach the elastic using a zig-zag stitch stretching the elastic evenly as you go. The fabric should not be stretched, only the elastic.
3. Flip the elastic to the wrong side. The scalloped edge of the elastic should now be on the outer edge with the fabric folded underneath. With the right side of the fabric facing up, zig-zag along edge stretching the elastic evenly as you go.