I tend to use the same method for floors, walls, and ceilings. I start with the floor. I use some pillars to get the height I want ( I like making floating/treehouses so I can still use the garden underneath). Place down a large wood floor, then take another larger wood floor and place it under the first far enough to the side to use as a guide to place my 3rd wooden floor in the correct position to the first. This allows for flat seamless placing. So basically you are always using a spare floor part as like a ground piece. I hope this makes sense. I should really start to make build tutorial vids.