Get a BIG prybar, and a helper to guide the assembly into place once you have it pried up high enough. I put the one end of the prybar centered on the control arm, and the bar under a protruding part of the knuckle. Once you have it in place a BIG rubber mallet can be used to persuade the balljoint to engage completely.
Probably not the best (safest) way, but after doing this crap about 5 too many times, I'm convinced it is the fastest and easiest. Be careful though, you don't want to let the pressure off the strut untill it is over the balljoint. I pinched the seal on a balljoint last time, so I'll have to do it again soon.
