In Parsvottanasana, or forward bending triangle you stand with one leg forward and the other back.
You bend forwards with your hands on the floor or with your hands behind your back in "Reverse Prayes." Hence this posture is sometimes called Reverse Prayer Pose.
Keeping your knees straight, you can use this yoga pose to focus on stretching your hamstrings one leg at a time.
You can also use it to open the front of your chest and shoulders if you put your hands in reverse prayer.
I'll focus here first on the hamstring stretching aspects of parsvottanasana.
You can read the reverse prayer article for how to work towards the reverse prayer hand position.
To get into triangle forward fold pose step one leg back. You can experiment with different distances from front to back but as a starting point have the legs about a legs distance apart from front to back so that your legs and the ground between them form a roughly equilateral triangle.
A wider stance is generally easier to balance in. You may also find that it is easier to square your hips to the front with your feet wider.
You could also position your feet so that your heels are on the same line. Or you can have your back instep inline with your front heel.
As you gain more experience in this standing yoga pose you can try experimenting with a narrower foot stance.
Lift the center of your inner and out arches and press down through your heels and the roots of your big toe and little toe.
As you activate your feet try to stack your heel bones (talus and calcaneus) so that both your inner and outer ankle joints feel open.
While standing upright, draw your ribs up away from your pelvis and open the spaces between your ribs so that your ribcage feels "open."
Pull your head back and up away from your ribcage so that your neck feels long.
Slowly bend forwards while keeping your spine long, and place your hands on your front leg or if they can reach, on the floor either side of your feet.
Keep both knees straight.
So that your hamstrings can relax, press your hands down onto your leg or into the floor so that your shoulders help to support your ribcage.
Then try to keep your hamstrings as relaxed as possible slowly bend your elbows. As you do so allow your pelvis to tilt forwards so that your ribcage moves down towards your front leg.
While bending your elbows, keep your neck and spine feeling long. You can even imagine trying to bend your spine backwards.
Bending your spine backwards helps to unify your upper body in parsvottanasana, making it act as one integrated unit.
Pressing your hands onto your leg or into the floor you can then use your hands to support this weight.
If you get to the point where you can lower your torso to your leg, or get reasonably close, you will probably find that you can relax your arms and take them off of the floor.
You can then use the full weight of your upper body to stretch your front leg hamstring without your hamstring tightening up.
You can add more weight to deepen the stretch by slowly reaching your arms forwards. Or you can put your hands in reverse prayer position (hands together behind your back).
Do you have difficulty keeping your hamstrings relaxed?
With your hands on your leg or on the floor, you may find it easier to relax your hamstrings if you engage your buttocks and the sides of your thighs and hips.
Try to slowly contract your buttocks and then slowly release them repeatedly.
You may find that you can tilt your pelvis forwards more as you release your buttocks. Or you may find that you can also tilt your pelvis forwards more while your buttocks are engaged.
In this case your buttocks are lengthening while staying active (eccentrically contracting.)
You may find that you can stretch your hamstrings in parsvottanasana by simply focusing on lifting or reaching your sitting bones upwards.
If you find it difficult to feel your sitting bones (the bones you feel when you are sitting on the floor or on a hard chair) you can reach the back of your buttocks upwards instead. As you do so focus on feeling or allowing the back of your thighs to lengthen.
You may find it helpful to roll your thighs slightly inwards or outwards in order to lengthen your hamstrings.
To add a muscle assist to this hamstring stretch imagine trying to lift the front leg.
Imagine trying to pull your foot off of the floor while keeping your knee straight.
To prevent cramping, try slowly activating and slowly releasing.
For an explanation on how you can might be able to get weight even on both feet (and why you may not be able to) read this entry in yoga notes.