Place the outer waistband (the one with the buttonholes) onto the trouser waist with the right sides together.
Match up the centre backs, side seam notches and centre front. Sew around the waist and press the seam up into the waistband.
Sew the two waistbands together around the upper edge making sure the centre back seams match up and the right sides are together.
Press the seam open and flat, then fold over the inner waistband so the seam is on the edge and press flat. Edge-stitch around the top edge of the waistband to prevent it from twisting.
Measure the elastic around your waist and add on 3cm. Sew the elastic into a circle by overlapping the ends by 3cm and sewing a square of stitching to keep the elastic flat.
Slide the elastic into the waistband and smooth the inner waistband down to sit flat underneath. Pin in place from the right side.
Sink stitch through the waistband seam so the stitching holds the inner waistband in place. You will need to gradually shuffle the waistband fabric round the elastic.