From the Main Menu go into Multiplayer and then choose "Play With Friends". The online for this game is likely to be a little dead, so your best bet is to boost it with a Friend. Therefore I recommend one of you creates a match.
Once created the host will automatically be requested to choose a character. Pick anyone you want and once you have done that use the
guide and invite your friend into the lobby.
Whilst waiting for your friend to arrive, you can change the settings by pressing the
button in the game. I recommend the following settings to be applied...Course - Seaside Hill - Whale Lagoon
Laps - 7
Private Slots - 7
AI Racers - Beginner
Power-Ups - On
Catch-Up - Off
When your friend arrives, press
to begin the race. Now you both need to decide who is going for the achievement. Once decided the method is to either let the person going for the achievement race around the track and use Power-Up's on the opponents around the track, or the other method would be for both of you to drive up to a set of power-ups, then let the person going for the achievement grab whatever power-ups they get and launch them onto the other player that is not going for the achievement.
If you choose this method, by placing the Laps at 7, it gives you and your partner enough time to work on this in a few races since the opponents will drive slow and will take awhile to complete 7 Laps.
This achievement will take a short while for you both to get, but once one of you gets it, swap over roles immediately. The achievement will unlock as soon as you have used your 100th Item in XBox Live races.