For this stretch you will need one (or two) pillows or soft cushions to place between your knee and the floor. You must be very careful when performing this stretch because it can be hard on the knees. Please be advised to take it easy (and not overdo) while performing this exercise. If you have problems with your knees, you may be better off not performing this stretch at all.
Put the pillow under your rear knee and let your knee rest on the floor. Lift up your rear foot and grab onto your foot with the opposite hand (grab the instep if possible, but if you can only reach the heel, that is okay). If you have trouble grabbing your foot, then you may need to sit (or shift) back onto your rear leg so that you can grab it, and then shift forward into the starting position (with your hand now holding your foot). Now, exhale and very gently, but steadily, pull your foot toward its buttock (butt-cheek) and lean toward your front foot (you may also wish to twist your waist and trunk towards the foot you are holding). You should feel a tremendous stretch in the quadricep (top right thigh) of the foot that you are pulling. If you begin to feel stress in your knee, then discontinue the exercise (but let your foot down slowly -- not all at once). Hold this stretch for about 15 seconds. When you are finished, shift your weight slowly back onto your rear leg and let your foot down while you are still holding onto it. Do not just let go and let your foot snap back to the ground -- this is bad for your knee.
Now for the isometric stretch: Get into the same position as for the passive quadricep stretch, but as you lean forward and pull on your foot, resist with the leg you are holding by trying to push your instep back down to the ground and out of the grip of your hand (but no actual movement should take place).
Now do the same stretch with your other leg in front.
Stop the stretch immediately if you feel pain or discomfort in your knee.