Sometimes guys have a tight foreskin opening. Many think this is phimosis, although it usually does not meet the clinical definition of phimosis. One way to widen the foreskin opening is to gently stretch it, as shown here. Stretching the skin like this a few minutes at a time for several weeks/months will allow the foreskin opening to enlarge so that the foreskin can easily glide over the glans.
See Doug’s manual tugging for phimosis treatment for an illustration of the technique with more instructions.