You need to use RO
water to calibrate it. You only need a few drops on the prism. Just enough to completely cover it when you flip down the plastic cover.
Look through the eye piece (may need to focus by turning the eye piece) and see where the line shows up. If using pure RO
water, the line should show up on the lowest possible reading which is probably a SG
of 1.000 and a salinity of 1 ppt
If it doesn't line up to those measurement, you may have to adjust it. Most have a srew on the top that you can turn to adjust the line up or down to get to a SG
of 1.000 and a salinity of 1ppt. It probably came with a screw driver to do this.