Please Help to move from Prototype to product

Hi
I have been working on Arduino projects, I have made the prototype but the size of the prototype is too big and bulky. I want to turn this prototype into a final product. I have searched on the internet and I have gone through many articles and blogs they were all about the PCB designing and all.
But I was not able to find How to use any Microcontroller unit in PCB designing?
Can anyone please help me with this?