I've tinkered with halo 2 but this is my first time on the classic. My question is how do I add doodads (rocks, trees, etc.) to my maps via spawn point or otherwise. I wish not to replace anything already present but what can I do to simply add something?

Please if at all possible could someone imput something on the basest level of this topic? I am also wondering if there is a way to extract/edit/inject custom models with which to use.