Depending on modelyou have, it may have a small knurled knob or yu may need a small screwdriver to adjust. After placing drop of RO or RODI water on window, andlowering the hinged plastic cover, make sure there is not any bubbles in the water yu have trapped. If it shows bubbles add more rodi to it, and try again. HOld meter up to light source and see where the blue and white portions intersect on the inside scale. Adjust it by looseing u screw or knob and make the blue and white intersection point start at the bottom of the scale or on "0"
Snug down knob or screw. Recheck and readjust if necessary. If its still at Zero, yur good to go, so just wipe off the window and hinged cover, and now add saltwater on the window just like yu did with rodi water. Hold up to light and look and where the two colors meet thats the salinity. ( I stated blue and white colors but I have seen green and white as well as other color combinations, so your really looking for the point the two colors whatever they are meet for the zero poinit or for the SG reading.)