Go on bungie.com/fileshare, then search machine, if you find the right map, you'll also see the game variant copy.

Click on the button "copy to Halo 3", you have to log into your account before you can download.

Then wait like 3-5 minutes and go on Halo, and you will see a message pop up in yellowish-orange saying the map is available in Forge and custom games.

Go in custom games and pick the map and the game variant and go up the stairs in the map.

Throw the grav lift and break the turret off and there you go, you got yourself a clone!