I did this:
Drain gearbox as I was fitting new passenger side driveshaft at the same time.
Disconnect gear linkage at the end of the long rod coming form gear stick one 13mm(I think) bolt.
Remove circlip from gearbox and pull downwards the plastic ball socket to remove small rod.
Remove or cut the old rubber gaitor on the selector shaft, if you don't discard this part and reassemble with it on you will struggle to get reverse gear.
Remove bolt holding the spring assembly and remove spring.
Carefully prise old seal from gearbox, then cut it off, sharp cutters required. Be very carefull not to induce any burrs into the gear box shaft or casting.
Clean selector and surrounding components thoroughly.
Pull the 2 rubber o-rings over the selector and work them up to the shaft where the seal will eventually sit, I put a bit of grease on them to aid them sliding along.
Clean again.
Put the 2 half circle plastic parts over the o-rings to make 1 complete unit. Then using the plastic shim supplied push the assembled seal all the way up into the gear box, I used a couple of large spanners as levers, work your way around gently do a little bit at a time and make sure it goes in evenly, If its all lined up you shouldn't need too much force.
Fit the metal retaining bracket supllied with new seal kit to the the spring bolt assembly and refit.
Then just stick it all back together in the reverse order of removal.
It comes with instructions although not the best, on the instruction sheet there is a link to the manufacturers website slightly more detailed instructions can be viewed there.
You will feel a firmness in the gearstick but this will return to normal over a period of a few weeks, mine did.
If you get stuck PM me and I will try to advise.
This may not make much sense until you get the kit in your hands and can see the parts that I refer to, should all make sense then.