Winegrowers Supplies - Hand held Refractometer - instructions for use
Lift the plastic cover plate, squeeze the juice from a single grape on to the prism surface, lower the plastic cover so that a thin film is formed on the prism. Point the refractometer towards strong light. Twist the eyepiece until the scales are in focus. Read off the scale at the white/blue transition, which shows the temperature corrected figure.
A drop of the stirred juice, after pressing, can be used to measure the natural alcohol %.
Use a table to convert °Oeschle (or Brix = % mas Sacch) to % alcohol.
Use a paper tissue to wipe the refractometer dry, do not wash with water.
Initially the refractometer will be calibrated correctly. You should not need to alter it, unless maybe you drop it. Then I suggest asking your WSB officer (when he visits you) to check against his model using the same juice, and if necessary adjust the recalibration/correcting screw using the tiny screwdriver supplied.
Alternatively you can compare the reading with that measured by a high quality hydrometer (with built-in thermometer), using the juice in the glass trial jar, then if necessary adjust it to the temperature corrected figure.
Do not, as a recent customer told me he had done, use water and adjust it to zero on the scale. The refractometer is not designed to be accurate at zero.