Unmodified Contact Error Solution Guide

I just went through this whole problem and I learned alot. I thought I'd post a guide with all the steps (from easiest to a complete wipe) I gleaned from research. So, one day you're trying to join a server. It loads fine, but as soon as you attempt to press "Join" it freezes and then this lovely error comes up. "This Server Only Allows Players with Unmodified Content. Please Revert To The Latest Official Version." Or something like that anyway. This begins what can be a long proce