First calibrate it. You can do this by adding distilled or RO
water and making sure it reads 0ppt or 1.000 sg
for distilled water.
To use (and calibrate), put a couple of drops of the sample water on the prism, close the lid so the prism is covered with a thin layer of water with no air bubbles (I usually use three drops spaced across the prism). hold the prism up to a light source and look through the eye piece and read. Make sure to set the diopter so the reading is sharp and easy to read