1 Spread open the sensor clasping mechanism by squeezing the rear tabs of the sensor together. 2 Place an index finger over the sensor window with the fingertip against the stop. Note: If you are unable to position your finger correctly or your index finger is not available, a smaller finger can be used. In some cases, if your fingernail is long, the nail tip can extend over the finger stop. 3 Turn on the oximeter, wait for it to perform a measurement and record the reading.