Assuming its an auto temp calibrated model...
Calibrate to 0.0 brix/plato with distilled or RO water that is roughly the same temperature as the wort sample. That's the ATC part of it...the calibration solution needs to be the same temp as the 'live sample' since large differences will skew the sample reading. Luckily you only need like 3 drops of wort, which is EASY to cool to 70F.
Always take a reading under strong light. I go right underneath a lightbulb for mine.
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10