step 1 open your options tabs on the bottom left , select the global tab( 2nd icon on the top row) then select the third icon guild forum
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step 2 you get the following screen
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click new forum
step 3 create the forum thread , name it and click create
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step 4 your done
if you are a leader or have guild rights you can do these steps if your just a member with insufficient rights contact a guild leader or someone who can enable those rights within your guild to do so