How do you write a book about fiddler crabs?

Answer 1

First you learn all you can about fiddler crabs.

Next, start writing down what you know. Organize it into chapters that make sense to you, like one chapter for how crabs eat and one for how they swim and so on - whatever sounds logical to you. Be sure to include a bibliography at the end of all the places you got your information so other people can read more if they want to.

Answer 2

Just start writing about fiddler crabs! Start explaining some things about them. You can write a book on any topic. :)