Firstly, you will need to teleport to Varrock, or "Home" as it is referred to by many players.
Once there, locate the Banker that is just west of the Other bankers, and bank booths.
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After selecting the "Set-Pin" option, you will get an interface in your chat box like so;
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Continue on with the options, selecting the ones you want.
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After selecting the options you want, you will see another interface on your main display that looks like this;
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Select "Set pin" and a screen like so will appear;
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After selecting your 4 digit pin number, a message in your chatbox will appear like so;
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If you follow this guide, you should have a bank pin set.
If you need further assistance, please either contact me ingame(E H) or use the ::help <reason> command to alert online moderators that you need help.
-E fucking H