If you have seen on youtube, and various other places one of the most common games that are modded on PC is GTA 4. I will be teaching you how to make simple mods from scratch.

If you have GTA 4 downloaded onto your computer, go to your Hard drive that has it installed. Click on it and go down to program files.
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Then go to Rockstar Games > Then Grand theft auto 4
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The first file shown should be common. Click on it.
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Then click program data. Scroll down until you find the file 'Weapon info.xml'
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Drag the file to your desktop. From the desktop right click on the file > Edit, and this code should pop up.
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Now that the code is open you can proceed to modding different things.