So how do you correctly stretch?

Try to allow 10-15 to correctly stretch your muscles.

You will want to stretch each muscle in your body.

One way to stretch your back muscle is the Pelvic Tilt.

Lay on your back and with your hands to your side and fit on the ground, knees bent you want to lift your butt off the ground pulling your pelvic off the ground hold this pose for about 15 seconds then go back down to the floor. You want to do this several times.

Neck stretching

Standing with your arms at your side and relaxed you want to move your head in a circular motion so that you move your neck muscles to relax them.

Arm Stretches
While still standing put your arms out straight and from your shoulders you will want to make circular motions with your arms.

Back stretches 2
On your hands and knees arch your back upwards to stretch your back muscles. Do this 10-20 times to loosen your back and stomach muscles.

Stand up and do bends at the waist touching your hands to the floor. This will not only stretch your back muscles, but also will stretch your leg muscles. Do this at least 20 times slowly.

While trying to stretch your muscles, you will want to breath deeply and clear your head of negative thoughts.

You can also do these stretches while cooling down after exercising.
Keeping your muscles healthy and flexible will make Yoga go much easier and you will be able to get the full effects of Yoga exercises.