This isn't really a step by step explanation but I'm going to explain this simply. An output can have as many inputs possible but an input can only have ONE OUTPUT(just putting emphasis on this one output, I'm not yelling). Example, the input "2" can be equal to the output of "4" depending on the equation the fuction, however, the input of "2" CAN NOT be equal to the outputs of "4" and "6" because as my Algebra teacher explained it "for every input, there is exactly one output". I hope this helps and I hope I didn't lose you in my explaination.
