I got from trying to touch my toes and only getting a bit past my knees to being able to go past my toes. In only a week! How I did this you ask? Did I have to spend hours stretching?
I was thinking about the advice to stop sitting so much, but I've tried that before and I run into problems with standing desks and all that. So I realized that what mattered was not that I was sitting, but that my hamstrings were short for extended periods of time.
With this in mind, I started sitting on the ground with my legs straightened. As this was a bit uncomfortable, so I only did it for ~45 minutes a day as I watched videos on the internet.
And now my lower body is rather flexible. My dead lifts are better than ever before, and the flexibility makes everything easier!