I just did this today, so I can give you a few words of advice. Every time you reset your diamonds, you lose all of the experience progress towards your next level + the amount from the previous level to your current. That means that to go from 18 to 1, it would take 17 games.
When the guide says you need to break the XP cap, this is true. You need to do this before you start de-ranking, as it is the key to the glitch. By breaking the cap, you will lose experience when you leave the game. When you get down to 0 xp at level 1, when you dashboard, you will lose all of the experience that was over the cap.
Just make sure you do all of it in one go.
Arcade games 100%
Retail games 100%
1,000th Achievement: A Monument to All your Sins (Halo: Reach)
10,000 Earned: Jan 2, 2010
20,000 Earned: June 28, 2010
30,000 Earned: May 30, 2011
40,000 Earned: March 17, 2012
50,000 Earned: June 10, 2012