It's difficult to explain but I've stripped and cleaned 2 * 500 and 1 * 4000 keyboard recently.
It's a struggle but can be done with patience.
I put one metal clip on the keycap, put the plunger/spring on and then got a small flat bladed screwdriver to assist with pushing the other metal bar into place.
The trick I found was to fit the metal bar to the keyboard and then slightly open the clip so it pops into the keycap.
Making sure not to exert too much pressure on the clip and bend it.
