How do you upload a forged map in Halo 3?

Let's say you forged Valhalla and called your custome named map "Sniper Spree." Go to Matchmaking and go to pick a map. When you scroll over to Valhalla it will give you a side selection. One will say Valhalla, and one will say "Sniper Spree." Pick "Sniper Spree" and you're ready to play your forged map. Turbo-117