Summarize this article:

Sit down with your back straight on a chair. Take a piece of paper big enough for your whole foot to fit on. Place your foot flat on the paper. If you intend to wear socks with the shoe you're buying, wear those socks while measuring your foot. With a pen or pencil, trace the outline of your foot. Keep the pen or pencil as close to your foot as possible. This will help ensure an accurate measurement. You'll get the most accurate measurement if you have someone else trace your foot while you remain seated upright, but it's okay to do it yourself. After you finish measuring your first foot, do the exact same process with the second foot. Feet are usually slightly different in size, so you'll be picking shoes with your larger foot in mind. Identify the areas on your feet with the largest width. Take a tape measure or ruler to measure both widths. The first measurements will not be entirely accurate. The pencil will have created a small amount of space, making your measurements slightly larger than your actual feet. To determine your most accurate foot width, subtract 5 millimeters or 1/5 of an inch from each measurement.

Summary:
Place your foot on a piece of paper while seated. Trace your foot. Repeat with the other foot. Measure the width between the widest points of your foot. Subtract to find your shoe width.