Stretching exercises for male? (Who has never bothered with flexibility)

After a long hiatus, more than a year of inactivity due to work schedule and lifestyle habits, I am finally trying to get back in shape, fitting in some form of workout to my daily routine.  

I got myself some dumb bells, an ab roller and a pair of running shoes. I'm trying to fit in some stretching exercises as I am also looking to improve flexibility but have no idea how to go about this. All I am doing is just stretching. I don't think it's working though. I feel like a robot, my hands can't reach the floor with both feet planted together, stretching is worst than doing bench press!

1. Do I stretch my hamstring one leg at a time or both legs at a time?

2. Do I need to follow a "stretching exercise" routine from youtube or I can simply stretch whichever way I want and as long as I feel pain/uncomfortable, I'd know it's working?

3. Any recommended stretching exercise/routine I can follow on youtube?

4. What were your motivations for achieving better flexibility and why? Flexibility is something I have ignored my entire life.. as I was always more of a "Upper body all day everyday" guy.