Formations

The hardest element to understand (stagger) is what is explained in this quick 3 minute video: https://youtu.be/NEdOJ6Jf-Jw 
As a general rule of thumb, left is prioritized for formations, this means that when forming a stagger, it is done from left to right, same goes for rank order, and same goes for any odd numbers in an arrowhead.
Shoulder To Shoulder (STS)