There are no instructions. Memory in all Retina MacBook Pros since 2012 is integral with the logic board and cannot be upgraded by anyone. Even Apple cannot add RAM.
However, since mac OS 10.9 back in Oct 2013 changed the way Macs handle memory, they can do quite well on 8GB RAM. I have two MBPs, on older and one new, that are quite capable with 8GB RAM.
Please do not apply obsolete ideas fostered by RAM sellers to the modern macOS. "Free RAM" is no longer the operative metric. The 2013 RAM scheme revisions stage unused RAM for faster access. You can use Activity Monitor (in your Utilities folder) to see the new metrics. If that utility show Memory Pressure in the green and Swap is zero or nearly so, your have plenty of RAM.
Activity Monitor User Guide for Mac - Apple Support
So if your computer is feeling slow, there are many other causes today than RAM. If you can tell us why you need more RAM, we may be able to help you diagnose the real causes.