I just recently stumbled upon a site called Derank.Me, in this short guide I'll tell you how you can use this site to rank down in CS:GO!
The first thing you'll want to do is login: (EDIT: Seams like you actually don't have to log in in order to join a lobby)
After logging in you'll want to find a lobby that's suitable! Make sure that your rank is high or low enough and that the lobby is fairly new (refresh the site every 5-10 seconds).
Quick explanation of lobbies:
No AFK: You have to play the whole game but lose on purpose
AFK only: You do anything during the game (Use one of the anti-afk methods mentioned below)
Win 3 rounds: You have to win exactly three rounds before you lose the game. (Apparently Valve stops counting losses if you lose to many games 16-0, that's why you'll sometimes have to win some rounds)
Then all you have to do is press JOIN!
This will place you in an actually csgo lobby where you only have to press accept when the queue pops!
Anti-AFK methods:
Fully still: Open up the console and write "+left", press enter, "+right", press enter.
Moving in a circle: In console: "+left", enter, "+forward"
Spinning in place: In console: "+left" or "+right" and then enter.
EDIT: You should also make sure that your max ping setting is as high as possible, you can use the console command "mm_dedicated_search_maxping 350" to set it to a pretty good value.
Good luck with ranking down!