nexuscore

hi all, i am new to modding and i want to learn how to mod like a pro ^^ what i want to do is mod the campaign so i can play as a grunt, but then comes the question: here are some lists of all the offsets for the things, but how do you guys find them?? i have done some hex editing in the past and some cheatengine too for pc games, but i really have no idea how to find the offsets here. anyone can clear this up for me? also, yesterday i tried to mod a halo 3 screenshot, so i opened a program and it was looking for my hdd but it said it retried 3 times and it did not find any screenshots then i opened my screenshots with a different program and then that program crashed. then i gave up and it tried to mod some films, i opened them and changed the armor: success i copied them to my hdd and then in game it said it was currupt or maybe for a& different version of halo 3 ...then i tried to mod a usermap, i modded in forge 2.5 and i put it back on my hdd and in game the map is not there anymore... sorry for so many things but i am new, please help me with this