Welcome to TheFiringLine!
First navigate to the portion/subforum of TFL that you wish to put your post in. (Please don't ignore/neglect this part of the instructions since this particular subforum/section (Site Questions and Support) is not a suitable place for firearms questions. It's only for questions having to do with how to use the forum software/features.)
Then look for the New Thread Icon
which is found on the left-hand side of the window at the top or bottom of the list of active threads. Click on it and that will bring up the editor window.
If you want to reply to a thread the procedure is not too different. When you have finished reading a thread, if you wish to reply to it, look for the Post Reply Icon
, (again it's on the left side of the window near the top or bottom of the thread listing) click it and then enter your reply and submit it.
If you are trying to post in the for sale forums, you should be aware that you must be a member in good standing for at least 30 days and have 25 MEANINGFUL posts in the other discussion forums before the forum software will allow you to post in that part of TFL.
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For more information about using TFL, check out the TFL FAQ