i'll tackle this one in a couple of parts, first off the floor! My interior set has wooden floorboards, I decided to make them out of balsa wood as they are easily drillable for tie downs, and easy to cut up into smaller scale appropriate pieces.
Before i bought the wood, i measured out the space i would need, and double checked by laying out the sheets to make sure that there was physically enough to cover the entire floor with a little extra on the edges.
I then cut all of the pieces into equal parts, something like 150mm by 25mm (don't quote me on that) resulting in a stop motion timber yard. The only issue with this though is to get the randomly placed quality they were all too organised, I didn't want any two pieces of the same wood to be touching each other (within reason)
So i had a little jumble! and all was well.
I then painted the base of the floor with a darker brown to the wood, this way if there were any gaps in the floorboards, it would look darker underneath.
Then it was just an incredibly long saturday spent gluing and placing each individual floorboard, trying to get them as straight as possible. I think i went through about 4 tubes of UHU (in a well ventilated area of course).
I then placed my props on there to get an idea of the final layout of the set. All looks a bit clean but I'm happy with them for now.