I want to attach the gear selector coupler to the gearbox, with a new dust cover. The old one was ripped and I tried to glue it together using an old piece of inner tire from a bicycle (a must have for various repair jobs), but because of old age (of the part, not me), I bought a new dust cover. I want to avoid ripping it when putting it over the coupler so I thought perhaps there is a way of doing this without ripping it.
Or I might as well put on the old dust cover and glue it together after putting it over the coupler. This is the easy way I guess.

How to?

 

The boot goes on first, then install the coupler. The boot will compress enough to set the grub screw on the coupler. Then light grease on the coupler to allow the boot to slide forward.

This is when you will need to stretch the boot some to get it around the coupler. If it is new, it will not rip.
Now set your shift lever index and tighten nut at spline joint. One you are pleased with the shift index, stretch the boot all the way forward until it snaps into the circular shape on the shift rod.

 

As Jim says, stick the boot onto the selector shaft first.

Then I just use a quick squirt of spray lube on the inner edge of the boot, hook one side over the grub-screw side of the coupler, and then get it over by using a combined pulling forwards and twisting motion which convinces the cover to ride over the other side of the coupler.

I usually struggle for a couple of minutes, then suddenly find the sweet spot and it just slides over easily.