In addition to your Murphy Cup/Float, you will need an hydrometer and syrup sample. The Murphy Cup/Float is designed to make your life easier but it still does not work without a hydrometer.
If using a Murphy Cup, float your hydrometer in the syrup sample in the Murphy Cup. Compare the hydrometer reading to the Murphy Cup reading. Return the syrup sample to its original container. Rinse and dry the Murphy Cup and Hydrometer.
If using a Murphy Float, place your Murphy Compensation Float and hydrometer side by side in a vessel of syrup. Compare the hydrometer reading to the Murphy Float reading.
Remove the Murphy Float and hydrometer from your syrup sample. Rinse and dry.
Allow both the dial and hydrometer to stabilize before taking a reading. And always start with a clean and dry hydrometer and Murphy Cup/Float.