1) Lay the completely buttoned & zipped trouser flat on a table face up, use your hands to gently extend and flatten out the fabric.
2) Begin by measuring from the edge of the waistband, down to the 1cm/0.5in point. Make note of this point.
3) Starting at the left edge, measure the waistband at the 1cm point noted in step two, all the way to the right side.