To use your thumbs to"help" the two stretching fingers in through the foreskin opening. Once you get your fingers fully through the opening, they will of necessity be lying beside the glans (penis head) so you will have them "hooked" in and they will not slide out. They must be fully in for you to know that they are actually pulling on the tightest part of the foreskin opening, which is that part immediately in front of the glans.
It sounds as if you need to do a little more work to get them fully in.
Have you read through the discussions here far enough to come across a description of the "balloon test"?
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