You’ll need a modified game client—basically a tailored copy of WoW that matches the server’s patch. The server’s website will link you to a safe torrent. Do not download from a shady pop-up ad that promises “FREE LEVEL 80 BOOST.”
I recommend a VPN here, not because you’re a hacker, but because it’s like wearing a raincoat to a water balloon fight. Only sensible protection.
Once downloaded, extract the files to a folder named “DefinitelyNotWoW” (for security, and also for the lulz).
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