Draw a rough sketch of an overhead view of the floor of the room you plan to build. Use a tape measure to measure an existing room if there's one that matches the desired size for your new room. Signify certain wall elements that will be included, such as windows, doors, electrical sockets and wall lighting; draw lines where these elements are desired.
Add in the placement of certain pieces of furniture that you plan to put in the room once it's completed. This will help you determine the overall dimensions of the room. Cut out furniture models from construction paper and spend time arranging them in the room sketch.
Transfer your rough sketch of the room onto graph paper, using a ruler (a triangle or T-square can also be used) to make straight lines. On paper, 1 square inch equals 1 square foot of floor space. A common rule of thumb to figuring out the new room's total wall area is to multiply the length of the room by the width of the room.