In all versions of this pose one knee is straight.
The other knee is bent with the shin folded to the inside of the thigh as opposed to the outside.
(In half hero pose or "triang muka eka pada paschimotanasana" the bent knee shin is folded to the outside of the thigh.)
In the "A" version of Janu Sirsasana the foot of the bent knee is placed against the inner thigh with the heel (ideally) close to the groin and the outer edge of the foot flat on the floor.
In the "B" position the foot position is similiar except that you sit on the heel of your bent knee foot.
In the "C" variation the sole of the foot is again placed against the inner thigh but the foot points downwards with the ball of the foot on the floor.
In all of these variations, you can bend forwards once the foot is in position.
Note that for the C variation, you might choose to remain upright until you get used to the position of the foot and knee.
When bending forwards you can a foot or wrist and then use your arms to pull your ribcage towards your foot to create space and length in your body.
Try to do this without compressing the hip joint of the leg that you are stretching.
You can use Janu Sirsasana A as a means of working towards the Seated Forward Bend (where both legs are straight).
To make it easier to stretch your hamstring in this posture you can start with the angle between your pelvis and your straight leg slightly more open (110 to 120 degrees). You can then work at reduicing the angle as your hamstring lengthens.
You can also practice opening your hip joint prior to bending forwards.
Opening, or "creating space" in the hip joint may make it easier to stretch your hamstring.
Place the bent knee foot against the inner thigh of the straight leg. If your bent knee doesn't naturally touch the floor, place a book or block under it so that you have something to press your leg down into. You can move the block outwards gradually as your bent knee hip opens up.
Sitting upright in janu sirsasana, you can practice creating space in the hip joint. This action may make your legs feel longer because you are reaching your thigh bone away from your pelvis.
Focusing on your straight leg first, reach your knee away from you hip socket each time you inhale. Relax when you exhale. The movement is very slight (a few millimeters!) but you may notice a pulling feeling around your hip. Do the same thing with your bent knee leg, each inhale "pull" or "reach" your knee away from the hip socket.
Work towards "opening" both hip sockets at the same time. Relax them together also.
Once you get the feeling of this action then focus on pressing through the heel and forefoot of your straight leg as you open both hips. Relax while exhaling.
Once you get the feeling of "opening your hips" you can also practice lengthening your spine and arms at the same time.
Practice lengthening your spine while sitting upright. As you inhale open both hips and slowly lengthen your spine upwards. Pull your head back and up, and lift your ribs away from your pelvis. Pull your chin in to make the back of your neck feel long. Slowly relax as you exhale.
Next add your arms, as you inhale lift your arms and reach them forwards while still sitting upright. Slowly lower them as you exhale.
The overall feeling of your body when inhaling is one of slowly becoming bigger. While exhaling the feeling is of slowly relaxing. If you move smoothly and slowly between these extremes (4 to 5 seconds inhale and exhale…. see if you can work up to 7 seconds) you may find the movement feels quite comfortable.
To turn the seated version of Janu Sirsasana into a hamstring stretch, add a pelvic tilt.
As you inhale tilt your pelvis forwards. Lenghten your spine and open your hips as you do so. Exhale and sit up straight.
As you tilt forwards focus on reaching your ribs and head away from you pelvis. Also focus on reaching your knees away from your pelvis. You can also focus on pushing your pelvis back.
Relax when you are sitting up straight.
For the next phase of janu sirsasana, keep the leg position and place your hands on the floor. Tilt your pelvis forwards. Also allow your spine to bend forwards.
The leg action is the same, but now straighten your spine while inhaling, bend it and allow your ribcage and head to sink down while exhaling.
Once you have that action, then lift your arms and then reach them forwards while inhaling, set them down on the floor while exhaling.
When lifting your hands, take your hands off of the floor gradually and slowly. Reach your arms forwards slowly and smoothly. While exhaling, place your hands on the floor first, slowly relax your ribs and head down.
Note that at first it may be "enough" just lifting your hands off of the floor. Lifting your hands up adds weight to the hamstring stretch and if you aren't used to this then proceed slowly.
Slowly relax your arms while they are on the floor. Then take them off of the floor. Then slowly reach them forwards to gradually add more weight.
Once you are used to reaching and then relaxing while in janu sirsasana, you can focus on first bending forwards with your hands on the floor. From there open both hip sockets, lengthen your spine and reach your arms forwards.
Look forwards and reach forwards with your eyes also. Hold this position while inhaling and exhaling.
You can focus on actively pressing your straight leg down into the floor so that the back of your straight leg is strong. Then, after 5 or 10 breaths you can slowly place your hands on the floor.
As you let your ribcage sink down you can focus on relaxing the hamstring of your straight leg and at the same time lengthening it by tilting your pelvis slowly forwards.
Another option is to allow your pelvis to tilt forwards while your hands are lifted.
In this case slowly lengthen your hamstrings while keeping them active and then, when you place your hands on the floor, continue to lengthen your hamstrings but focus on relaxing them at the same time.
Note that if you can't get your pelvis past vertical when trying to bend forwards, bend your straight leg knee and/or sit on books or a block or even a chair.
If using a chair, you may need to place a support under your straight leg knee.