Dunno if they were Britpart or not but I've fitted a couple of sets over the years. I made up a template from cardboard or something fairly solid, then measure it carefully into place, level etc. As with everything, " Measure Twice, Cut Once". How spot on you want to be depends on if you are planning on fitting SW inner trim ?
With the template marked onto the panel drill a hole just inside and cut out with a jig-saw. No need for a grinder. Tip - Put duct tape or similar on the sole-plate of your jigsaw to reduce damage to the paintwork.
Offer the window up, tape it loosely in place and drill out the holes for the rivets - I used m4 countersunk machine screws & nylocks as the final finish inside is nicer. Remove window, run a bead of silicone glue/sealant round, re-insert window and bolt/rivet to your hearts content.
The "finisher" strip can be a PITA. Insert the main bit, joint at the bottom then feed in the thin bit that expands the finisher. The professionals have a tool like a wire triangle on a handle. A blunt screwdriver works OK. Washing up liquid as a lubricant makes life easier. Don't cut the inner bit of the finisher until you are nearly done - it stretches/shrinks as you work and you'll not get it right until the end. Cut it with a cm or two extra and feed that in to keep the whole thing tight.
Good luck !
