How to write inputs for American Fuzzy Lop?

I am trying to use American Fuzzy Lop, so far I got everything installed and trying to fuzz "as an example for a popular program" the program Putty-gen.

However, I'm having a problem in determining the test cases, and the difference of files between a program that reads input from user on run time, and a program that reads arguments.

Can someone please explain in short the difference between the above? and give an example on how to write a simple test case for the program putty-gen or similar?

I am testing the program on Ubuntu CLI, not GUI.

Thank you.