Hi Edgar
You shouldn’t be aiming at stretching the skin. The objective is to stretch and expand the sheat (fascia or tunica) surrounding the CS/CC, as well as the CS/CC itself. Only then will the volume increase. You can feel it as a tough layer just under the skin when stretching your penis. It is often referred to as milking, and the purpose is to trap the blood temporarily and expand the CC and the CS, thereby stretching the tunica.
Stretching focuses on lengthening the ligaments that attaches the penis to the pubic bone. I sense, that the gain many see in the beginning is from this area. It will also to some degree lengthen the fascia/tunica.
Again, it takes alot of dedication to get worthwhile gains, and possibly some light exercises to do, even when you get to where you wanna go.
One last thing: Get properly measured, and warm up.