Basically 5-15min of stretching in the morning and evening (shorter if younger or decently flexible, longer if older or less flexible). You start with what you can do easily then go until you "feel it" then you do your best to hold it for 10-30s and relax (slow breathing). If you stop feeling it go farther (it was just muscle tensing in the way) and a couple inches more and you'll be at your current real limit hold for 10-30s then do other stretches.

Will take 1wk to a couple months. The biggest caveat is the risk of injury for those who don't know their bodies well or try to rush the process.

Isn’t it just a matter of slowly acclimating yourself?

Get adopted by a ninjutsu master, train your entire childhood, then compete in the kumite for your family's honor. Watch out for Chong Li, that fucker cheats.

I didn’t.

I've tried for years through yoga and stretching. I've even taken classes on it from contortionists. Nothing. There does seem to be some biological aspect to it, I stretch nearly every day

https://old.reddit.com/r/flexibility/comments/9212b8/dont_know_where_to_start_click_here/

Splits

This splits routine was created for the 90-day challenge and will give you quick results by stretching every day.

If you just want to take it a bit slower, here's a follow-along video for every other day.

Hit a plateau in your splits training? Try these brutal but effective loaded progressions. Here and here. Oh, and here.

For front splits: Very deep lunges to stretch hip flexors, hamstring stretches, then get as close as you can, preferably supporting your crotch ('sit' on a small stool, then a yoga block, whatever supports you in the position) so that you can relax into the pose.

Best to do these after a workout while you are warm, then again before bed when you are cool, and go to your edge but not past it, that edge will move.

For straddle split, I don't know if everyone can get those but my nearly 60 year old husband can, and mostly he just lifts weights, so maybe. Those you can practice hot or cold, and if you do the widest you can and lean forward for 5 minutes it will also relax you for sleep. So good practice for bedtime.

you need to start rather young, like under 10. if you're past a teenager its probably impossibly painful. theres videos online for training up to it.