For removal, it helps greatly to have a nylon O-ring pick. You put the point down in the groove under the ring and spin the pick to hook the ring and lift it out of the groove. If it's an outside O-ring, get it started and gently roll it off the part with minimal stretching, unless you're sure you won't re-use it.
I'll go as far as to say a pick is mandatory to remove an inside O-ring. Be extra careful with metal picks that you don't stab the ring and cut or damage it, unless you're sure you will be replacing it. Smooth out any sharp edges or points on your pick.
For installation of an outside O-ring on a cylindrical piece, O-rings have to stretch significantly and slide over larger surfaces before popping into their groove. It helps to pre-lube the ring generously over its entire surface *before* installation to help it slide in place and to lubricate the ID of the ring where it seats on the gland. Installing them dry and lubricating the exposed surface only is not good practice.
When installing an outside O-ring, any sharp edges on the part they have to slide over will easily cut and break a stretched O-ring before it seats. You place the O-ring on the end of the part and gently roll it in place while trying to minimize stretching. If this doesn't work, use a tapered, cone-shaped object, with the flat end of the cone pressed against the part that has the groove. Lube the cone and the ring, and slide the ring along the tapered cone to expand the ring and get it started onto the part.
The 'witches hat' device is the tapered piece, an apt colloquial description I guess, except you obviously don't want the brim. Make one out of a piece of artist paint brush handle, dowel, or other handy object. I supposed you could improvise one from a rolled-up piece of thin brass shim stock for a custom application.
A similar method is helpful to install new piston seals on spring airgun dovetail studs.
Regards,
Feinwerk