Ah fitting out a camper from scratch - been there, done that!
Firstly does your Kombi already have cupboards and furniture in it, or is it an empty box? What layout are you going to use? Even factory Sopru campers had at least a dozen variations, and aftermarket converters had many more.
In my case, I used the basic layout of my Sopru's original design, but which was made from crappy laminated chipboard that was worn out. I measured everything, then ripped the lot out. When came the time to rebuild, I designed everything from scratch with help from my Dad, a retired high-school woodwork teacher.
We rebuilt the insides in several steps:
1. Clear out the entire rear, scrub clean and repaint so it looks like new.
2. Re-lay the floor in new 12mm plywood and apply new vinyl.
3. Work out how far the sink cabinet will extend out from the right wall, then design and make the rear seat mounting appropriately. Make sure the seat extends up and folds down.
4. Design the right rear clothes cupboard using cardboard templates. Make in 12mm marine ply and install.
5. Measure the sink, and work out how much space you have for the right front cupboard. Design and build, and set in place.
6. Fit the sink in between, then design and make the centre right cupboard to mount it.
7. Design and make the fridge cupboard for the left front.
8. Design and make the rear upper storage compartment.
9. Remove all, stain and estapol, then refit permanently.
if you have some interior in it you can do as i did. if the frame is sound, work out what bits of it are usable and what needs to be changed on the outside dimensions (for instance i lengthened my bench, moved the stove and sink around into spots i found better), then deside what needs storing, install shelves/drawers etc onto the frame and doors last.