A general technique for stretching tight hamstrings is to practice stabilizing your legs and then relaxing them.
You can then work at stretching the hamstrings while the legs are relaxed.
To get a taste of activating and relaxing muscles we'll start with sideways or transverse yoga poses.
For the standing side bend I'd suggest contracting the short side of the body. If bending to the right, push your hips to the left and contract the right side of your waist, ribcage and the right side of both legs (the outer right thigh and the inner left thigh.)
You can focus on smoothly contracting the right side and then smoothly relaxing. Then focus on smoothly opening the left side and then relax repeatedly. Then try both actions (contracting the right side and opening the left side) simultaneously.
To add an arm stretch to this pose reach the arms up over the head. Try to make them feel long. Each time you activate the sides of your body try to draw the arms towards each other.
Because the feet are parallel and facing straight ahead this pose may not affect thigh hamstrings. But it can be used to warm up the hips and it gives you a taste of both creating space and contracting or stabilizing.
Moving on from standing side bend the next stretch for tight hamstrings is triangle yoga pose.
With feet wide and knees straight, turn your right foot out 90 degrees and your left foot inwards. Tilt your pelvis to the right carrying your upper body with it. This may add a slight stretch to the hamstring of the right leg.
You can help rest your legs by placing your hand on your leg and pushing down with it so that it helps support the weight of your upper body.
Slowly relax you arm and as you do so you may notice that your legs become tense in order to support your upper body. Memorize the feeling of your legs tensing and deliberately tense them to support the weight of your upper body. Your arm should relax naturally as a result. You can then lift it and reach it to the side. Then put your arm back down again, use it to support your upper body and to add a stretch to your hamstrings bend your elbow to allow your chest to move closer to your thigh.
Repeat this a few times each time allowing your ribcage to sink a little bit deeper.
So that your legs work harder during the active phase you can reach both arms to the side.
Another "sideways" pose is half moon yoga pose.
Keeping your pelvis facing the front turn your right foot outwards 90 degrees. Shift weight to your right foot and touch your right hand to the floor. If you can't reach the floor either bend your knee or use a yoga block (as shown in the pictures.) Lift your left leg and reach your left arm straight up. As for half moon pose you can practice making your foot and leg feel strong and with your weight over your foot, lift your hand.
If you are using a block work at reducing the height of the block so that you gradually get your hand closer to the floor.
If you have your hand on the floor work at gradually increasing the bend in your elbow so that your ribcage moves closer to your thigh.
Wide leg forward bend can be a good stretch for tight hamstrings because it can be relatively easy to get your hands to the floor even with tight hamstrings.
For the active portion of this exercise use arm strength to pull your ribcage towards your thighs. You'll probably notice that your legs will activate also. Relax and repeat a few times ideally drawing your ribcage a little closer to your thighs each time. Rather than using momentum try to move slowly and smoothly.
Once you get comfortable using your arms, try putting your hands on your waist and use your hips and abs instead.
For better hamstring and glute control in general, it can help to work on activating these muscles with the hip extended.
Balancing on one leg, you can practice extending the other leg. Rather than just reaching the leg back, start by bending your lumbar spine (and optionally, also the thoracic spine) backwards using your spinal erectors. Then extend the leg and as you do so focus on contracting your glutes and hamstrings. Keep the knee straight while doing so.
Moving into a lunge position with one foot forwards and the other back, bend the front knee and rest the back knee on the floor. Spread your legs far enough front to back so that your front shin is nearly vertical when viewed from the side.
You can start with your hands on the floor and practice straightening your back knee. Try to use your glutes and hamstrings to straighten the knee. Rather than focusing on pushing your foot down focus on pulling up on the knee so that your glutes and hamstrings engage. Next try the same action with your torso upright (hands can rest on your front knee.) Start with the knee on the floor and toes tucked under. Pull up on your knee to straighten it then slowly relax.
Finally, after straightening the knee, use your spinal erectors to bend your spine backwards.
Forward bending triangle is perhaps one of my favorite postures for stretching tight hamstrings. Stand with your right foot forwards and left foot back. Turn your back foot out slightly so that you can keep your heel on the ground. Try to keep your hips facing the front. Keeping the knees straight bend forwards and if your hands can't touch the floor rest then on your shin or on yoga blocks or on the seat of a chair (ideally one that doesn't have wheels!)
Relax your arms and notice the increased tension in your leg (particularly the front leg) as you do so. Now start by bracing both legs, and then relax and then lift your hands.
Put your hands down, use them to support the weight of your body so that your legs can relax. Repeat a few times with a focus on activating and then relaxing the legs. Once you can feel your legs relax, you can then work at letting your chest sink closer to your thigh while your legs are relaxed. See if you can let your chest sink a little further down each time.
To move into revolved (twisted) triangle lift your right hand and turn your ribcage to the right.
Here again you can practice bracing your legs prior to lifting your bottom hand off of the floor. The challenge can be in maintaining the twist while your hand is lifted.
This set of actions both strengthens and stretches the calfs.
Bending forwards one more time, with feet about hip or shoulder width apart and parallel, place your hands on the floor (or on yoga blocks or a chair) and lift your heels.
Work at lifting your heels as high as possible and then straighten your knees. Pause and then release and repeat a few times (at least five.).
Next, switch and lift the fronts of your feet. (Again, you can use a chair or yoga blocks). Lift the fronts of your feet as high as possible. You could shift your hips back relative to your feet to lift your forefeet higher. Instead try to move your hips forwards relative to your feet. And focus on decreasing the front angle of your ankles. Try to pull your forefeet towards your knees. And if your knees have bent while lifting your forefeet, straighten them.
Pause and then release and repeat a few times.
This last position, lifting the forefeet, is difficult, but it stretches the calfs and the hamstrings together. In addition, becasue the tendons of one of the calf muscles, the gastrocnemius, interlocks with those of the hamstrings, this may help to give the hamstrings a stable foundation from which they can then stretch. And so as you get used to this forefoot lifting action, you can work at deepening the forward bend of your hips by tilting your pelvis further forwards.
If you like you could alternate the two actions.