cr0sh:
Mount the end cap in some kind of solid holder (a large vice is a good choice) and heat it up with a propane or similar torch. Melt the solder into the cap so that you have a "pool" of solder to dip into.
Alternatively, there are small solder pots available at fairly reasonable prices (like the first two in this link). Not as cheap or adventurous as cr0sh's suggestion, but muc easier to work with if you are going to a sizable batch at the same time.
cr0sh:
Clean your copper rings, perhaps dip/wipe in flux - then dip the rings in the solder (you might have to fashion some kind of tool to allow for this, of course).
Or you could just buy a extra large pair of hemostats and wear leather work gloves.