While both the previous solutions are basically right, I didn't feel they had quite enough detail. Apologies if this over simplifies it (but as they say, the devil is in the detail)
1. Select a multiplayer game
2. Press X to add a second controller
3. Select local/split screen match
4. Have your main controller as a buggy on team 1 and the second controller as the turret on team 2.
5. Select no bots and start the match
6. With the turret shoot at a green turret upgrade icon.
7. With the first controller drive the buggy over a red turret downgrade icon. BINGO!! Cheevo acomplished.