The process is pretty simple.
Patience and lots of it. Seriously template everything, I just use plain old cardboard. Then cut 1/2" cabinet grade plywood cutting all pieces as snug fitting as possible.
The plywood is wrapped in a material called soft and sable, I purchased at a local fabric store. It is applied with a high quality spray adhesive.
The toughest piece is the floor of he trunk, it's what everything references off of. A trick I found was use spray foam under this once you have the piece cut. You only need to put it where there is cavities or to help with making the floor parallel with the trunk rail and as close to the same in height from side to side,(just makes it easier for templating.
Here's a picture of a kit laid out on the floor for my 68 GTS.
Every car is different and they all have there challenges. But what a difference it makes in the end. Cheers Bob