Step 1: Lay the trousers on a flat surface.

Step 2: Fold the top leg above the waistband, exposing the crotch seams. Use your hands to gently extend and flatten the fabric.

Step 3: Begin from the crotch seam, and measure down 45 cm/17 in point. Make a note of this point. 

Step 4: Beginning at the left edge of the pant leg, measure across at the previously marked 45cm/17in point. Be sure to avoid any slack on the measuring tape.