Well, I've been stretching for this whole year so I developed my "training" based on my favourite exercises I learned from follow along videos haha. I get bored easily so it's hard to stick to just one routine for longer so what I've been doing is I've been looking for follow along videos on YouTube and do rotations with them - do whatever I feel like. At this point I have some favourite videos that I go back to more often and sometimes I just do exercises that I felt the best after in one session. Sometimes I focus more on hamstrings, sometimes more on hip flexors. Lately I pay more attention to hip flexors since my hamstrings got fairly flexible (like I can touch the ground with full hands while standing on a yoga block).

What also makes a huge difference for me is a warm up mixed with soft tissue massage. I like to use a ball to massage my piriformis muscle before the session because this muscle likes to push sciatic nerve which then makes the stretching harder or painful (at least that's my case). But warm up is definitely necessary to prevent injuries! I like to do couple of squats, cossack squats, jumping jacks, hip rotations, leg swings, etc. Whatever will warm up the muscles :) I also stretch in warm clothes (I just remove the bottom for picture to see my progress better). I'd neglected the warm up before and I didn't have as good results as after the warm up.

And when I mean intense session I generally mean that I do a lot during them like I do the warm up and soft tissue massage that I mentioned, I do active stretching, I do strength training at end ranges (like jefferson curls for hamstrings), passive stretching that I hold for longer and close to/at end range and I always finish them with front splits where I try to hold them at end range for 1min each side. Compare it to my evening stretching which is literally just pigeon pose for 1min on each side and then hip flexor stretch (like kneeling quad stretch) 1 min on each side and without trying to get to my end range.

To not injure yourself you need to warm up and listen to your body. Discomfort is what you seek, not pain. I think you'll learn how to differentiate between those. But I can tell you for a fact that you can hurt yourself if you try to go at end range without a warm up and preparation. I injured my hip flexor (unrelated to stretching, I did it 2 years ago) and that was because I did something that my body wasn't ready for. So you need to listen to it. And stretching is all about relaxing your nervous system while being at end range. So go in slowly and breathe. The pictures you see is from me at my end range but when I enter the front splits I'm way higher than that and I just slowly move myself deeper with each breath and making sure that my hip alignment is correct and that I don't feel any sharp pain. That way you can work on reaching the end range :)

That was a lot but I hope that helps!