Creating a floor plan is an important step when starting on many projects. Planning is always the best advice when deciding on what furniture to purchase along with what sizes, what fits, the scale of items, etc. Once you have drawn the walls of the room, it's easy to play around with the layout to incorporate furniture, lighting, area rugs and any other furnishings. It then becomes a shopping guide to go out and make your purchases.
Measure the length and width of the room. Start with these measurements first to make sure you have the overall square footage of the room correct. Then measure the areas between doors, windows, hallways, built ins, and fireplace.
Draw the overall size of the room (grid paper or a computer program is very helpful). Be sure to note locations of doors and windows.
Add in the furnishings at the exact size. Refer to catalogs and online sources for product descrptions of sofas, chairs, tables, etc. Figure sofas about 32"-36" wide and the length can be a 7ft, 8 ft, or 9ft, it just depends on the configuration. Allow 3 feet clearances for circulation. You can also make cut outs to play around with layouts.