Oh, I have to remember !
The easiest way is to do it with the whole assembly out of the car.
The way I did, is that I held the shifter in place, with the side pin in the lever, and slid the two buckets through the assembly and over the bearings, then put the clips.
Then finish by tightening the set screw gently (there is no need to use a lot of force here !), and tighten the locking nuts.
The buckets should be tight inside the assembly so there is no play. If they are too tight, a little bit of 400 grit sand paper should clear it
It should work by doing so.