I have a set on the Derby alright.
You deflate the tyre (don't break the bead) and you push the tyre wall out of the way a bit while you squeeze the insert into the space between the tyre wall and the rim. You get about an inch done at a time, working your way around and around and around until the trim is seated properly all the way around. You need to go around a few times to make sure it's all home and then inflate the tyre. I had one of the trims pull out on a particularly high-speed motorway run on the way back from Tipperary Racecourse. It was the first day out with them on so I'm not sure it was seated 100% but once home I deflated the tyre and re-seated it. I've been paranoid about them since but it seems if you can keep the speed under warp factor 7 they'll stay put. I've been driving the car to work and around and about the last few weeks and they're fine.
Obviously they come to suit different diamater wheels so make sure you get the ones to suit your wheels. I got mine second-hand but unused. You can get them as a set from SSP for about £40ish.
one thing though, all the guys I've spoken to about whitewall flaps say that they eat the sidewalls of the tyres. Now whitewalls are quite soft where the redbands are quite rigid. Only for the tyres on my derby are low-profile and the trim doesn't rub the sidewall that hard I'd be worried (I've been watching for that too). The guys I'd spoken to all had full height bubbles on their vans and the sidewalls were being eroded by the trims. Some people recommend lubricating between the trim and the tyre wth vaseline but it splatters and looks bad. Just bear that in mind.