i put oil on the shifter and the rubber bit and then just pulled it down over the top, then wiped off any excess at the end.
also i found that fitting the large metal spacer inside the shift casing didn't fit first time. I had to get a file out and file off some burred edges in the grooves of the gearbox shift housing. It is just from wear and tear. But the difficult thing is doing it with out letting the filings go into the gearbox. So what i did was put a bit of oil on the small file, and some on my finger... i'd give it a bit of a file and then wipe the inside clean and also wipe the file, then i'd put a bit more oil on the file again. This just helps the metal filings stick to the file so they don't drop into the gearbox.
Pictures 9 and 10, where you are putting the spacer in, show the gaps i'm talking about. Roughly halfway down where the metal splines from the plastic bit sit in those gaps were worn a little and stuck out just enough so the spacer wouldn't fit all the way down.