I think it helps to first figure out what needs to be stretched.
Try to stretch the way you want, and FEEL what muscles and ligaments are under tension. Trace those muscles as far as you can, maybe look up their names in an anatomy chart, and figure out what exercises use them.
You may discover it isn't really your hamstrings or quads that need stretching! For me, on almost any stretch, I feel the stretch in my inner thigh - I think mostly my psoas. Though that's unusual.
It's important, because it is the muscles that you want to stretch that you need to warm-up (exercise) first. Following someone else's stretch regimen, adapted to their personal needs, may be useless or even harmful. You need to balance the flexibility of various body parts for what you want to do - else all the force is concentrated on one body part, which is easily damaged by accidental over-stretching. I've done that. So work out your own stretch regimen. Consult a Sports PT or Trainer if you are afraid to.
If you can't feel what is stopping your bend, muscles that aren't next to your skin are hard to feel. You may need to do enough exercises on the right muscles to make them sore in order to feel them.
If it feels like a joint (bone or ligament) is stopping you, or if the stretch is in your neck or spine, there may be no good healthy solutions - consult someone medical.
Why would an expensive stretch cord be better than cheap hardware store cord?
Some people's muscles stretch easier than others. 10 or 30 seconds won't stretch me AT ALL. 10 or 15 minutes, re-adjusting things every couple minutes, is about the minimum I need, and most of what I achieve is gone by the next day. IT'S NOT FAIR!, but life never is.
You might look up PNF stretches, but be careful.
I would like a magic pill that gives me the body I want today. Or a machine I can walk into, select the desired body parameters, and push a button.