First wind your bobbin with elastic thread. You’ll find the thread in a package usually on the notions wall. You want to stretch the thread slightly as you hand wind it (most machines won’t be able to spin it properly).

You want to start with about double the width of fabric then you will need. Then just set your tension really high and your stitch length really long.

All you have to do is sew make sure to do extra back stitching to secure the sewing. I use the presser foot as a guide to keep my rows straight but if you want the lines more spread apart you may want to use a disappearing marker and mark your sewing lines before you begin because once things become bunchy it’s pretty hard to just eyeball it straight.

Use an iron to steam the shirred area. Don’t hold the iron directly on the elastic (I’ve melted it before). Instead tap it over the fabric side. This will make the fabric shrink up (which is the goal).