Since there has been a patch that only allows you to get 1 Combat Engineer ribbon per round on horde, I'll show you how to get around it.
YOU NEED TWO CONTROLLERS
1. Go to the dashboard, System Settings, Memory, Highlight hard drive, press y, Clear System Cache
2. Load up Gears of war 3, sign in on controller one with your main account and your 2nd controller with your other account. it doesnt matter if its gold or has any xbl membership.
3. Go to Horde and when the game starts, have player 2 give all its money to player 1.
4. Buy a command post, player 1 buys the turret, player 2 gets on the turret and keeps shooting with the turret while player 1 stands next to it and keeps repairing it.
Once the round starts go back to the main menu and repeat steps 1-4
You can get about 10-20 Combat Engineer ribbons each round doing this