In Upavistha Konasana (seated wide leg forward bend) the legs are open about 90 degrees with the knees pointing straight up.
You can do this pose with knees bent if your hamstrings are tight. However, whether knees are bent or straight, work at keeping them pointing up as you roll your pelvis forwards.
Prior to bending forwards in upavistha konasana, you might find it helpful to warm up your hips by shaking or rolling your legs from side to side. I like to do this while sitting upright with legs apart.
To Relax Your Hamstrings
Use Your Arms for Support
You can use your arms to support your body when you are trying to get your hamstrings to relax. Imagine that you are doing a pushup, using your arms to support the weight of your upper body. So that you stretch your hamstrings, gradually lower your body, as if lowering your body into a pushup. So that you can keep your hamstrings relaxed, do this slowly.
Create Space in Your Hip Joint
You can also try creating space within your hip joint to help your hamstrings relax. Create space by reaching your knees away from your pelvis.
Widen and Lift your Sitting Bones
To help your pelvis tilt forwards, experiment with widening and lifting your sitting bones.
Open Your Legs, But Not Too Much
In upavistha konasana, you can focus on stretching your hamstrings by opening your legs to about 90 degrees. If you want to work on your inner thigh flexibility then work to widen your legs further.
90 degrees is important especially when trying to lift your hands off of the floor. It gives your body leverage when you press your legs down.
To that end sit with your legs open a maximum of 90 degrees from each other. Sit with your knees pointing straight up. If you need to, bend them or sit on books or blocks or even a chair.
Reach your knees away from your hip sockets.
Do this whether your knees are straight or bent.
Tilt Your Pelvis Forwards
Keep your hips open, place your hands on the floor in front of you and tilt your pelvis forwards. Use your arms to support or brace the weight of your upper body so that your hamstrings can relax.
Once you have tilted past about 45 degrees you can pull your hands in close to your body and then slowly relax your arms and take your hands off of the floor.
You may notice an increase in hamstring stretch as you lift your arms off of the floor.
If you are even more sensitive, you'll notice your hamstrings stretching more even when you relax your arms prior to lifting them. This is because as you lift your arms, your body will no longer be supported. The unsupported weight of your body will then tilt your pelvis forwards, assuming your keep your upper body stiff.
Reach Your Arms Forwards, Slowly
To add more weight to upavistha konasana, slowly reach your arms forwards but keep your hands off of the floor.
You may be able to get your chin on to the floor. If that is the case, keep your arms lifted and try to reach your chine and chest forwards, away from your pelvis.
Alternatively see how close you can get to the floor without touching your chin or arms (or forehead) to the floor.
Even though your chin is on the floor you can imagine lifting your head so that the weight of your head helps to pull your ribcage closer towards the floor.
If you put your hands on the floor you can imagine lifting your arms so that they still help to sink your ribcage down.
Keep Your Knees Pointing Up
As your pelvis tilts forwards, concentrate on keeping your hips open. Also keep your knees pointing upwards.
When you are ready, (or have had enough) place your hands on the floor. Keep reaching your arms forwards but also focus on letting your upper body sink downwards.
To come out of upavistha konasana, sit up slowly. To make sitting up more of a challenge try sitting up while keeping your arms reaching past your head.
I've included a more detailed step-by-step guide to lifting the arms in wide leg forward bend.