What kinda' mods do you want to do? If you just want accuracy mods, you only have a few things to change.
1. Extend the frown to a point
2. Replace the ear screws with accurate ones
3. Replace or modify the mic tips
4. Remove some of the excess plastic around the eyes
None of these are required for 501st approval, only Centurion.
Whitearmor.net is typically your best resource for detailed tutorials. That thread I linked previously has some helmet mods detailed in it, I believe.
If you want to add electonics, padding or modify the lense mount, I can show you what I did for mine, but for the most part there is no 'right' way to do those.