Wear eye protection! Place the u-joint in a vice, put some grease in the bushing channels, grip the clip crossways tightly with a needlenose pliers, and place it firmly in position on the bushing. With a hammer, tap it in firmly - and hopefully - with a single blow. Did I mention that you should wear eye protection?
If the spring steel clip should come loose and fly away you may have a helluva time finding it. I learned to tie some fine monofilament fishing line to the clip to retain it in the event it takes off, then use a propane torch to burn away the line afterward.
