Measure the picture -- unless there is a border you want showing, subtract a 1/4-inch from all sides -- and write down the length and width. Determine the amount of mat you want showing, if any. If you want 2 inches all the way around, you need to add 2 inches to all sides. Add the measurements together to get the entire length and width.
Determine what the finished size of your framing order will be. If the picture is 8 inches across and you want 2 inches on the left and right, the total for the width is 12 inches. Do the same for the height. It can be easily done by drawing out the sizes of the picture, matting and frame with the measurements of each written down. Use a calculator if needed.
Add the two sides -- width and height -- together. If the width is 12 inches and the height is 20 inches, the total would be 32-inches. If the resulting number is a fraction or an odd number, round it up to the next highest even number -- UI charts go by even numbers only.
Use the chart given by the framer to determine the amount of molding, matting, backing board or glass you will need. Any site that asks for the UI either will have a chart or will determine the amount based on the information you give them. The thickness of the molding can also determine the amount and price of the supply.