First, cut all the 2-by lumber before you start construction. Cut the first piece of a given size, and then use that piece to mark the dimensions of its mates. A carpenter’s pencil or lumber crayon is handy for marking the cut pieces.
Second, you want the walls to fit the actual frame, so don’t cut the plywood until you’ve assembled the framing and measured it. This compensates for the inevitable minor assembly errors, and your finished house will be all the better for it.
