The elastic I used was 2cm wide - you could use wider elastic but you'd have to change the dimensions.
Make sure that the smaller piece is wide enough to fit the elastic through it!
Turn both pieces right side out and press flat, making the seam central to the back. A pencil may help turn the pieces inside out!
Using a saftey pin to help you, thread the elastic through the narrower tube of fabric. Pin the elastic in place at each end, leaving all the gathers in the middle. Stay stitch across the end about .5cm from the end to hold the elastic in place.
Lay the larger pice of fabric flat with the seam on the bottom. Lay the elasticated fabric on top right sides together (the seam on the top piece will face up). Line up the ends and wrap the lower piece of fabric around the elasticated part.
Sew across the end taking a 1cm seam allowance.
Trim the seam to .5cm
Repeat for the other end.