Help me with flexibility

After years and years of sitting in front of a computer I have poor hamstring flexibility.

The thing is i've been lifting weights pretty regularly for at least 8 years now and have good numbers on squat, deadlift, bench. I do a lot of romanian deadlifts and kettlebell swings.

But these don't seem to help with sitting at 90º with my legs straight.

I can search for flexibility routine, but the internet these days are full of ad riddled and generic content that I'm having a hard time filtering through the bullshit and finding something that says "do this 3 times per week, progress like this, etc". They just throw some stretches at you and don't say exactly how to progress and what to look for.

It's not like lifting weights that you put more weight on the bar to quantify things easily.