The trick is to remove the window. It's a 10 minute job, if that, which you do from the inside of the car by lots of firm but gentle pushing, and easing the rubber seal back over the metal bodywork using something to hold the rubber off once you start to get under it. I used a couple of small screwdrivers and some long nosed pliers. You'll be surpised how easily the wondow comes out.
Once the window is out, you slot the new trim into its place in the rubber. It's then just a case of fitting the window again. You achieve this by using the "rope method". Some strong string will do, but you thread the string into the channel in the rubber seal that folds over the bodytwork, position the window in the aperture from the outside, hold the windown in place with one hand, and start to pull one end of the string so that the rubber starts to fold back over the lip of the wondow aperture. Sounds complicated but it's not really (and I'm generally hopeless at these things). Do a search on Youtube and you're bound to find someone demonstrating the rope method.
Don't be fazed - it's an effort but easy DIY and very satisfying when you crack the job!
Let us know how you get on!