Measure Out a Route to Use for Calibration
The first thing you need is a known-distance course. Most outdoors tracks in the USA are 400 meters around the inside lane (this is about 1/4 mile), and that is a choice for calibration.
If you don't have access to a track, you will want to measure out a route where you can walk or run nonstop that is at least 1/4 mile.
If you plan to move the sensor between shoes, especially if they are not Nike+ ready shoes, you should recalibrate the sensor each time you switch it between shoes. Slight variations in the tilt of the sensor can affect the accuracy of the readings.