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Members who did not create an Author profile during registration need to add Author privileges before creating an Author profile later on. This is intended to bring attention to the terms and conditions that apply to Authors, especially those pertaining to file uploads and copyright. Simply go to your ACCOUNT page (via the main menu). Next to the Authors heading will be a link to the relevant terms and conditions, and another link to the Add Author Privileges page. On the resulting page, you will be able to actually activate Author status. There is a "go back" link for you to use if you change your mind (or you can escape through the main menu without doing anything.)
After activating Author status (which implies agreement with the terms and conditions), you will be taken to the Author profile creation page. Enter your name (or your pen name if you use one) for this profile, select its visibility, and click the submit button. This will take you to your (mostly empty) Author profile page, which you can then edit in the usual way, adding a bio, headshot, and editing the existing elements, the same way as any other profile.