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