There are various tutorials and resources that explore and explain the PBR process far better than I ever could. Instead of trying to explain things myself, I thought it would be better for everyone if I just pointed to some excellent Physically-Based Rendering resources and let you have a go at it that way. 🙂
PBR Theory by Marmoset:
This article covers some of the theory behind the PBR process.
PBR in Use by Marmoset:
Takes a more hands-on approach to PBR including the whole Physically-Based Rendering creation process.
Unreal Engine and PBR:
The folks at Epic talk about a basic PBR workflow with Unreal Engine.
Unity PBS Breakdown:
Unity uses what they call PBS or Physically-Based Shading, and this is a good read on their approach.
CRYENGINE PBS Information
Like Unity, CRYENGINE uses a Physically-Based Shading workflow or PBS. There is some good information here about what needs to be done.