It is always nice to get some idea of "where you are at", and given that you are quite young (certainly in comparison to ourselves), we can be all the more reassuring about your anticipated success.
Just retracting your foreskin is not
going to be very useful. Even when erect, the glans is far too soft to apply anywhere near
enough pressure to stretch. You need to use your fingers (which are - as you will easily realise - much
tougher than your poor old glans) or construct the stretching device
Yes, the entire
skill you have to develop, is that of getting the fingers inside (or at the very least, getting a single finger large enough to be a good tight fit) the tight part. Tight
is the word, and the reason that the glans is ineffective for this purpose. It is
a skill, and you simply have to master it; once you develop "the knack", the job is half done as it were.
I'll tell you what - the pictures we would
like - we have plenty of pictures of sad-looking penes with tight parts and such - would be of someone successfully performing the two-finger stretch, just as in the diagram you cited. Mind you, if it takes two hands to do the stretch, you would need a "helper" to take the photograph, wouldn't you?