Here is a step by step tutorial explaining how to sew fold over elastic to any neckline.
Sew the elastic to the body between the points on the front.
Align the groove in the elastic against the cut edge of the top. The elastic should be sewn to the top wrong sides facing. Line up 4" (10 cm) of fabric and elastic a time, and stretch the elastic a little bit as you sew.
Fold over the elastic and sew it to the outside with a straight triple stitch (or a three-point zig-zag).
Trim both sides so they line up with the fabric.
The straps and the rest of the elastic is sewn to the body in one operation.
Using the strap template from the pattern measure one strap and mark the length with a pin.
Fasten the elastic to the front point wrong sides facing. Leave the strap unfastened and start at the marked pinpoint.
Sew all the way to the other point, measure and cut at the end of the other strap.
Starting with the left strap, fold over the elastic wrong sides facing and sew all the way around to the end of the right strap. Use either a three-point zig zag stitch and a straight triple stretch stitch.
Try on the top, and find the correct length of straps for your body type.
Sew the straps to the back according to notches on the pattern.
Courtesy of Lone Thomsen / Sewn By Lone