[QUESTION]How to install mods for games?

I want to install the Mod Manager for GTA4, but I really don't know how to lol... I look up tutorials on Youtube and Google but can't find a DAMN thing...

They're all talking about how to install JB's with USB's and how to use the Mod Manager and stuff...

Can some give me a quick tut on that please? thank you!
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Seriously? Out of the 50 viewers, NO ONE HAS ANYTHING? Wow wtf...