They sell this tool (hope the pic comes through) which is designed to put it on.
It costs around $50. The basic premise is to spread the cap open to fit over the fence. When we put ours on we just got our players and some screwdrivers. Put one kid on one side and another kid on the other side. They put the screwdriver inside the split and pull apart. You can get a third kid involved following behind pushing it down or let the two with screwdrivers use their free hands to push down as they go through.