Hurdlers stretch gets a bad wrap because it can be quite dangerous for the knee. The trick to protecting your knee (particularly the bent knee) while doing hurdlers stretch is to do it with awareness. That means moving into the pose and coming out of it slowly.
As to why you might want to do Hurdlers Stretch in the first place? If you are a hurdler it will make it easier to jump hurdles. But even if you aren't, it's a good way to work on both hamstring and hip flexibility.
Plus it's one of only a few poses that help you stretch your hips with the thigh internally rotated. So if you do a fair number of externally rotated hip stretches like pigeon pose, Janu Sirsasana A, lotus, hurdlers stretch will help you balance those externally rotated thigh positions.
Another reason for doing hurdlers stretch is that the shin is actually externally rotated with respect to the thigh. When the knee is bent the shin can rotate with respect to the thigh. In positions like lotus and janursirsasana a, the shin is rotated to the inside of the thigh. In poses like hero and this pose, hurdlers stretch, the shin is rotated towards the outside of the thigh.
To keep the knee safe while doing hurdlers stretch it can help to make the knee active. One way of doing that is actively pressing the foot into the floor. Rather than just leaning into the foot to press it down, use the muscles of your leg to press the foot down so that you get a feeling of muscular (and connective tissue) tension in the leg.
One way to position the bent knee foot in hurdlers stretch is to have the inside edge of the foot on the floor (first picture below). This results in a deeper (more severe) rotation of the shin relative to the thigh. If this is too much for your knee, try placing the top of the foot on the floor for a slightly less intense shin rotation (second picture below).
This article includes two versions of hurdlers stretch.
Note that if you are jumping hurdles, then you aren't doing it passively. Your legs don't magically float up into the hurdlers stretch position. You have to use muscle power to get your legs into position and that's one way you can look at approaching hurdlers stretch. Instead of sitting in it passively, get active. Being active in the pose can help you protect both your knee and your hip. Plus you may find it helps you to move deeper into the pose.
But first some tips for getting into it comfortably.
First of all, if you do have knee problems, then move into this pose with a lot of care.
If you can sit upright comfortably with your legs straight in front of you, then one method for getting into hurdlers stretch is to lean away from the leg you are bending. Bending your left knee you can use your right hand for support.
Leaning onto your right hand you can use your left hand to help bend your left leg into position. Position the bent knee leg so that the thighs are open at about 90 degrees to each other. In addition, turn the foot of the bent knee out so that the inner edge of the foot is against the floor. And as mentioned, here's the first modification. If this foot position puts a lot of stress on your knee then reposition your foot so that the top of the foot is on the floor.
Another option for hurdlers stretch is to sit on a yoga block. If doing so, be careful to avoid pushing your straight knee down, since there is nothing below it. Or stick another (ideally lower) yoga block beneath it.
If you do have to sit on a block to do hurdlers stretch, you may find it sufficient to simply stay in this position with your spine upright. Focusing on your bent knee hip, you can try pressing the knee or the foot down. Experiment with both to see if either one makes your bent knee hip more comfortable. You could bend forwards towards your straight leg. Here also play with pressing down through knee or foot of your bent knee leg.
To make your hip more comfortable in this position, try pressing the bent knee down into the floor. Doing so activates some of the muscles of the hip and you may find that simple act is enough to make your hip a little less uncomfortable. You could try pressing your foot down into the floor also.
Sitting upright, one way to get used to this pose, particularly if your bent knee hip tends to lift is to shift your body towards the bent knee hip and use your body weight to press the hip down. Move gently and rather than forcing the hip down, move only as far as comfortable then shift away from your bent knee. Repeat this action and as you do so, you may find that you can gradually sink your hip deeper.
Combining this with the first action, as you shift your body towards your bent knee, actively press your knee down using muscle power (as well as body weight.)
As mentioned, there are two options for bending forwards that I like to use when doing hurdlers stretch. To stretch the hamstring of the straight leg bend towards the straight leg. To focus on stretching the hip of the bent knee, turn towards the bent knee 45 degrees and bend forwards between your legs.
This second option can be quite excruciating and you may find you wonder to yourself whether you are hurting your hip or stretching it.
If this second option feels to painful (or you are worried about damaging your hip then leave it out. For myself I went through with it and now find this position reasonably comfortable. However, it may have caused some problems aswell. These problems (clicking in the hip) can be surmountable with a lot of work on body awareness. And I'm not positive that doing hurdlers stretch caused the problem. If you are just doing yoga poses for the fun of it (or because someone told you they are good for you) then leave this out. If you are interested in exploring your body, or you do a lot of activities where flexibility is important, then go for it, but do so with awareness.
Generally I start with the first option, bending towards the straight leg. You can make this action more comfortable by placing your hands on the floor and using them to help support the weight of your body.
To train your hamstrings and glutes, press your straight leg into the floor. See if you can use your straight leg to support the weight of your body. Your hands will relax and then you can lift them. To exercise your hamstring more (and increase the likelyhood of increasing hamstring flexibility) reach your hands forwards. Hold for a moment, then put your hands down and relax the leg. You can repeat this a few times, gradually getting used to using your hamstring to support the weight of your body while bending forwards in hurdlers stretch.
Another option as you get more comfortable is to bend forwards with your hands lifted or behind yoru back. Go slowly and focus on feeling your leg and using it to control the descent of your body. (It may take a bit of practice to work towards this.) Use the strength of your leg to lift up out of the pose also.
One approach for bending forwards between the legs is to start by bending towards your straight leg, then keeping the forward bend, slowly sweep your body towards the bent knee.
Another option is to turn 45 degrees towards the bent knee and with hands on the floor bend forwards using your hands do help support the weight of your body.
To make the bent knee hip more comfortable you can experiment with pressing the knee down or pressing the foot down. As with the hamstring stretch, try pressing both the straight leg and bent knee down so that both legs help support the weight of your body. Then try to lift your hands. Then to add more weight, reach them forwards.
As with the hamstring stretch you can also try bending forwards without the help of your hands.
To make this pose more like an actual hurdle position, try creating an upwards pull on both legs. Imagine pulling them up off of the floor.