For a digital scale, you need to power it on and set it to your preferred measurement unit. Once the scale shows a Zero with the measurement you chose, you hook the fish onto the scale. Usually, the fish is hooked from below its gill. For a digital/ electronic scale, you can hook multiple fish. Check the weight on the screen shown.
For a mechanical scale, you first need to ensure your scale is at zero, and that the fish is drained of all water. Check the weight that is shown and remember it, as it won't be saved, like that of a digital scale. You can also only measure one fish at a time. Once you're done weighing all the fish, you can check the weights you've recorded and calculated the total weight.