Lay the trousers flat on a smooth surface
Smooth out any wrinkles
Measure from the top of the waistband to the bottom of the hem for full length
For inseam, measure from the crotch seam to the bottom of the hem
Use a flexible measuring tape
Keep the tape straight and aligned with the seam
Record the measurement in inches or centimeters
Measure both legs if you want to check for uneven hems