Getting the bearings in place is not a problem after you spread a generous amount of grease on the fixed cup's race. You can insert the spindle from the outside of the cup to act as a guide and then use a pair of long tweezers or needle nose pliers to press each bearing into the grease, they'll stay put while you remove the spindle and install it from the correct side.

You can also drop the bearings into the grease from the open side if you lack the correct tools, but if your aim isn't good you stand a chance of bouncing one into a frame tube, and then you'll have to start over after you fish it out.

The whole operation is easier with both cups out of course, you can bed the bearings into the grease and they'll stand quite a bit of handling without falling out.