Level 100 requires 3,851,750 experience to unlock. The easiest way to boost your experience will come from earning (and re-earning) Onyx medals - see "Seriously 3.0" for a full list and guide for those. There are many medals that can be quickly boosted by yourself (some require two controllers), though you'll get a ton of experience working on the other achievements in the game and the other DLC as well. Different methods work for different people, and some just like to earn experience as they play the Versus mode normally. Do what feels right for you and try to make it something you'll enjoy so this isn't a complete annoyance.
To physically "re-up" once you have hit level 100, go to the "Stats and Awards" screen and press .
Note: If you have already reached Level 100 prior to owning this DLC, NONE of the experience you earned after that will carry over to your second tour of duty when you choose to re-up. You will start back at Level 1 with nothing.