It's quite easy, actually...most mods come with some sort of installer that unpacks the mod directly into your GR/mods folder. If the mod is packed with win.zip, win.rar or some other file compression tool, the safest bet is to unpack the mod into your GR/mods folder, but there's always a read-me file that explains exactly how to unpack/ install the mod. Once installed, you have to start GR and activate the mod trough options/mods buttons, then select the desired mod and hit activate button.
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THX Streinger your a star