Tutorial (Xbox)
1) Join a friends player match
2) Get him to search for a ground war game
3) Make sure you join a pre-game lobby
4) Leave the lobby without party
5) Back out of the player match
6) Go to Local, Press (Y) on system link and press (X) to join back
7) Create a class, choose a perk set and press (X)
Buy and equip the perk you want!

Tutorial (PS3)
1) Join a friends player match
2) Get him to search for a ground war game
3) Make sure you join a pre-game lobby
4) Leave the lobby without party
5) Back out of the player match
6) Go to Local, Press (Triangle) on lan party and press (Square) to join back
7) Create a class, choose a perk set and press (Square)
Buy and equip the perk you want!