How To Catch Fish Using a Net

First thing is First, make sure you are standing in water, catching fish on dry land isn't going to work out too well.
starting note: you have to be careful with a net, it is easy to drink water with an empty hand, this is explained below.
Start out by drawing your net in a hand, then all you have to do is hit space to catch fish.
you need to keep hitting space. like with a fishing pole you will hear fish splashing around as you do this, then when you catch some you will hear a fish noise.
when you catch fish the net will be taken out of your hand and you will then have a net with captured fish in your inventory.
this is where you need to be very very careful.
if you are hitting space too fast, and hit it after you catch fish, you will then be hitting space on an empty hand while standing in water. this will drink water directly from the water source you are standing in, and doing that can give you mindthurst. this is a very nasty virus that has killed allot of people. the only way to get rid of it is via antibiotics, or by drinking allot and allot of water.
now hopefully you don't have mind thurst. you want to go into your inventory and draw your net with captured fish.
Use alt+T to combine net with captured fish with an empty hand, this will take your captured fish out of the net. both the net and the captured fish will then be in your inventory.
the benifit to the net is it will catch multiple fish at once, making it very easy to catch lots of fish.