Start with the foundation, outside edge to outside edge, and triple check the measurements. Everything else is based on those measurements. Save yourself some time and trouble and measure more than the affected area. Include the whole house if possible
Use a tape measure, you can get an appropriate sized tape from a hardware store relatively cheap. On the inside it is faster to use a laser measuring tool, but cheaper to use a tape measure. If you are using a contractor, make sure he verifies your measurements before he begins work or any mistakes will be blamed on you. If you are doing it your self measure two or three times. Make sure you locate and mark all plumbing and electrical outlets on the plan.
Last, no amount of measuring will uncover everything in any renovation. So don't get too upset with whoever you are working with when surprises happen..