Most guides I’ve ran across lump basic stretching, I.e. Stretch and hold for like 30 seconds in each direction, in to the “Warm-up routine”. I assume that is how the original poster knows it. To answer the question though, stretching is to well.. Stretch. The best thing I can do is when you’re stretching just feel around the base of your penis and make sure that your ligs and such are being stretched.
In my experiences if I pull too hard I start getting an erection which pretty much null and voids the stretches, just gotta wait for it to go down of course. Though you’d be surprised how little you really have to pull to stretch what needs to be stretched. I’ve been trying to perfect it as stretching too far, I’ve been told, will cause a natural reaction of your muscles pulling back and hence forth shrinkage.
To re-iterate, pull as much as you need to feel the stretch. It’s rather easy to feel if your ligs are being stretched and it doesn’t take that much at all to do it.