I agree with Nate152. You only have to mod a case if something does not fit right.
Otherwise you are modding your case for cosmetic reasons.
There no one sole material or technique used for case modding.
Skills involve for case modding includes but not limited to metal working, painting, model building, working with plastics, minor mechanical, basic electrical, and drafting and design.
I suggest you look up other case mods and see how they are done. Then look into the appropriate skills for that particular mod.