Sorry, but that diagram is wrong
It suggest exactly the opposite
of what we need to explain.
While it is indeed true that you wish to apply the stretch specifically to the tight part of the foreskin opening, you need to note that the tight part of the foreskin is not
the tip of the acroposthion
, but the inside
extent of it, sitting right against the tip of the glans.
In order to stretch this, you must
get your fingertips (or possibly cotton buds/ "Q-tips" or a "dilator") through
the narrow part in order to apply outward tension to it. Since this narrow part is sitting against the glans, then getting whatever you are using through it, automatically means that the object will have to slip beside the glans unless
you have firstly stretched the foreskin strongly away
from the glans by pulling on the end; and kept
that tension once you have the stretching device(s) inside.
Where you are
using fingers, the trick will be to get at least the first finger inside the space between glans and foreskin, and "hook" that finger outwards and forwards so that it continues
to pull the foreskin away from the glans, both to prevent the finger slipping out, and to avoid the discomfort of the finger pressing on the glans. Considering the sensitivity of the glans, you would prefer some lubrication on the finger (or certainly when using a "Q-tip") since what is actually irritating to the glans, is friction to which it is simply not familiar.
These are the two problems which fellows have with starting the stretching process - getting the fingers inside which requires the foreskin to be stretched forward
away from the body in order to slip the fingers in, and keeping
the fingers in place which is actually much easier when they are fully
through the tight part. The diagrams which imply that you can stretch simply by getting the fingers into the very tip of the foreskin opening, are actually a hindrance more than a help in that respect.