Like the front wheels, part of the 3 piece rims goes inside the tire. While it was tricky getting the piece inside the front tires, it was finger-numbing and near impossible to do it for the rear tires. I got help from someone with stronger fingers than mine and even he had a hard time. But once that was accomplished it was time to screw all the parts together.
Unlike the front rims, the nuts for the rear rims must be deeply seated in their holes. Just dropping them in place doesn't work every time. They'll end up sideways or they won't go deep enough without some prodding from a small screwdriver (or the end of the tweezers). If not seated properly it will be impossible to get the screws in so spend some time making sure all the nuts are flat and tapped into place before starting to screw the rims and tires together.