If you're doing the job inside and have the time then you should cut the window holes and then line the van, that way you can carpet up to and over the window opening lip before sticking the glass on. In this case you would not need to use the knock on edging.

However if you want to put the windows in quickly then put the trim on as recommended and when it comes to carpet lining, just leave a few mm's of spare carpet around the window hole and tuck it in under the knock on edging. It is a bit more fiddly but can be done with patience. One of the tools in the trim removal tool kit is very useful for tucking the carpet in as it has a curved plastic end.