Essentially they are nets that hang vertically in the water and are called gill nets because fish swim into them and become entangled by the gills, fins etc.

You would either "hang" them in the water by attaching something that floats to the top part of the net and something that sinks to the bottom part of the net, or you would stake them across a stream and anchor the bottom of the net with rocks.

After that they can either be left alone and fish will just swim into them naturally and become entangled, you can "herd" fish into them by basically splashing around like a nutcase, or you can lure them into the net .

Basically the net is left hanging like a curtain in the water instead of being dragged through it.