Most sacs can be slipped on only using your fingers, without any other assistance, but there are some awkward sections where you need a bit of help – necked sacs, for instance, or where the section protrudes a long way, or when there’s a breather tube.
When I began repairing pens (it was quite a while ago!) there were no specialist sellers of tools for the fountain pen repair-person. Then, you adapted tools made for other purposes – still a good solution for pen repairing. I found a pair of dividers in an old geometry set, snapped the points off and filed the rough edges away so they wouldn’t tear the sacs. It worked very well and I used nothing else for many years.
More recently, I bought this handy gadget:
Actually, it’s more clumsy in use than my dividers, but there are occasions where the wider sac opening it gives can be useful, like when an especially thick section protrudes well beyond the nipple.