hammer and a socket. Find a socket that's just a hair smaller than the cap. I like to put to hammer on the ratchet end of it, so I don't flare the socket end. But preferably, I use an old spare.
After you get the new U-joint in, take the hammer and hit the end of the yoke a few times on each side to relieve any tension. when the U-joint rotates easy, you're good to go.