how to hack halo and trainers

hey i have a couple questions. first, i have no idea how to install halo mods that are on the net. i downloaded one and it is a .dds. do i use halo cache editor or something? can someone please either explain it to me or point me in the direction of a tutorial? also, i would like to know if i download a trainer for halo and i play halo from the dvd drive, will the trainer permanently hack halo or can i enable and disable the trainer?

edit--never mind about the trainers. i went to the ones already on evox and you can enable/disable them.

edit again--ok i think i've figured out how to install patches from this guide. http://halomods.com/tutorials/Basics/ppf/ppf.php but that's only for .ppf's. what if the file is say a .dds?