http://h2v.halomaps.org/index.cfm?pg=6&sid=23
Install map editor with the original Halo 2 game
Grab h2codez run it let it update,
Make a map in blender (better) , or 3dsmax8
Setup the textures etc in the map editor
Export the map in .ass format
Use map editor to compile it.
Load the scenario in sapien setup spawns, game type flags settings
Save and package the map and then play it
Very basic tutorial of my version lol