Please excuse the dodgy photos - I'll retake them when I can do it in natural light & see if I can get better shots!
First of all, measure your child.
DD measured 50cm
Calculate and cut the fabric
The elastic I used was 2cm wide - you could use wider elastic but you'd have to change the dimensions.
Cut the elastic 22cm unstretched (20cm plus 2 x 1cm seam allowances)
Cut two pieces of fabric 32cm long.
The one to cover the elastic will be gathered by the elastic and needs to be twice the width of the elastic plus 2 seam allowances - in my case 6cm wide. (Not 7 as the picture shows!)
The second piece is 6 times the width of the elastic plus 2cm - so in my case this was 14cm
Press all the fabric so it's nice and flat.
Fold both pieces in half, right sides together and sew a 1cm seam along the long edge to make a tube.
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.
Turn the hairband the right side out and stretch the elastic to even up the gathers.
Finished!