Before I injured my back a couple years ago on a work related incident, I was the guy that you would see suspending myself over various inanimate objects in full splits or jumping in and out of em and all that crazy fu...
Now there are probably safer ways to go by especially at your age (and possibly fitness, dunno where you're at, I always played sports so kung fu wasn't a shock to my system as it was for others)...
but my general stretching routine was...
1) Start with joint rotations if you aren't already warmed up. If you finished a workout and are warm cool, if not get the joints movin'
2) I would just hang my upper torso down. Really completely limp and sorta sway. Legs straight tho, just don't pass out haha...would do that for about a minute or two
3) Leg swings, like kicks but keeping your leg straight. Now I know what most people are thinking, ballistic stretching is a big no no. But I have honestly never been injured or seen anyone injured by them. If you are mature enough to know your limits and start slow and low, then gradually work up higher as you get more limber then I see no reason not to do em, especially since dynamic flexibility is the goal yeah?
4) More torso hangs, this time pull down try and touch the ground or grab your ankles whatever. I'd do a few rounds of that for bout 10-15 seconds each. Dunno that's just what I did, maybe doin it just a while straight is better...
5) Then I'd do back bridges. Dunno if that's within your capabilities. If not, just lay on your back with your knees up and do pelvic thrusts. Going back to what Ronin said, hips and back....
6) Side to sides like you would see runners do...I do about 30 of em getting slightly faster toward the end
7) Only then after all that I did splits. Then it was center, then side and other side. Did about 2 or 3 sets each depending how I felt.
After I felt I was reasonably stable in splits to either side, not necessarily flat but stable and it didn't hurt, I started doing reps of tension in the stretch. Building strength in elongated position...that's way down the road though....
Now as I said, check out some the stuff others have posted. That's just what I did to get my flexibility...just be smart and be careful whatever you do...nothing sets you back like an injury...and I'm still young enough to "pop back up"
Oh and as far as the stretching machine goes...just save your money yo...
When you get in a center split try to just relax your muscles. Stretch your arms out in front of you on the floor and just slowly try to "walk" out with your hands. Relax and breath...there's your stretching machine experience.