To check the waist on trousers you already have please lie them flat, don't bring the front of the waist up to match the back
Lie them flat & measure across the very top of the waist inside the waistband that is & double it to get the waist measurement
Please keep in mind different styles of trousers sit differently & the rise makes a difference too
To check your inside leg length please lie 'skinny' trousers flat & measure from the crotch to the hem