Part 1: SketchUp UVs
Before you begin it is important that you have SketchUV extension for SketchUp installed. You can grab it from the Extension Warehouse and install it.
Part 2: PBR in Unity
For more info on PBR (Physically Based Rendering) textures it is advised to look at Allegorithmic's PBR Guide Vol#1 and Vol#2 to understand in more detail the science behind PBR.
Part 3: Textures in PixPlant
For the final part it is recommended you use PixPlant for seamless texture and render map creation. It is extremely powerful and easy to use.