It's very smelly! I'm making a batch of liquid krill right now & it stinks. All you do is get a barrel with a lid. Bung in the fish scraps & cover with water & leave to rot. Hence the lid
It helps if it has a tap on the bottom of the barrel. I make liquid noxious weeds as well. Saves me worrying about them resprouting in the compost pile. You can do liquid road kill, if you are really brave.