You can access the customize menu by standing on the plot and double clicking the house sign. It's under customize/customize this house I think (not in game right now). This should give you a selection of walls, floors, doors, etc. At the bottom of the menu there are options to clear (remove all walls and floors from the plot), save your current design, etc. There's an eraser to the left of that and a eye dropper to copy a tile you've used already so you don't have to hunt for it again. You may want to turn on tool tips so when you mouse over things it tells you what they are/what they do.