Lay the jeans flat on a smooth surface
Smooth out any wrinkles
Find the crotch seam where the inseams meet
Measure from the crotch seam straight up to the top of the front waistband for front rise
Measure from the crotch seam straight up to the top of the back waistband for back rise
Keep the tape measure aligned vertically
Record the measurement in inches or centimeters
Compare the measurement to the jeans size chart if needed