1. Choose a cleanhard, and smooth surface at home, preferably a table.
2. Place a straight stretched measuring tool on the table.

Waist:

Take from the closet pants or jeans that fit well both in length and width. Button the pants button and zip up the zipper, then shift the front of the pants about 10cm to the side.

 

Firmly stretch both upper edges of the pants' waistband, read the waistband cm and multiply by 2 (video example results - 49cm x 2 = 98cm at the waist (corresponds to size W36))

Hips:

Place the tape measure so that it touches the ends of the side seams and the lower curve end of the zipper's decorative seam (see image). Measure the hip width from one side of the fabric to the other (not to the seams).

Length:   

Similarly, do the same and measure the length of the pants' outer side seams (together with the waistband).