Most of the time, I just snap them on with my thumb, but as we all know, every so often you run across a mould that has an oversized GC shank and getting the GC onto that shank takes more than thumb pressure. In the past, the way I dealt with this was to place a small piece of 1/8" flat-stock over the sizer die and use the nose-punch to push the bullet down into the GC. This works fine (if the bullet isn't too long), but it's kinda slow.
Recently, I found a pair of well-used channel lock pliers at a pawn shop that a previous owner had taken the jaws to a belt sander and ground off all the teeth, leaving the jaws smooth. They cost me all of 50 cents, and it turns out that they work quite nicely for seating GCs onto recalcitrant shanks.