Create a concept. Some artists take months or years to finalize the concept for their installation. Ideally, your installation idea should be coherent and make sense. There are no limits to what you can do in installation, but as a general rule it must be interactive. For example: You could create an installation of simple audio recordings of fish sounds or as complex as turning an entire building into the inner organs of a fish.
Create a plan for your concept. Include all the important information about your installation, such as building plans for platforms, sculpture designs, measurements, materials and cost lists, and audio or video scripts, as well as anything else that may be of importance.
Acquire the funding to create your installation. Most artists pay for their own installations, but some are fortunate to receive grants and scholarships to create their pieces.
Execute your plan to create your fish installation by building it, recording the audio or video, making the sculpture or whatever else may be necessary to make your installation a reality. Remember that you are free to do anything you want and call it an installation as long as it is interactive.