As the Cardinal Flies

You need to be logged into your AO3 account (that would be the one you created back during Yuletide); then simply click on the "Post New" link at the top right corner of your screen under your account name, and you'll get the relevant Web page.

Note that if you are posting text from MS Word, there is a magic button for handling the paste portion of the cut-and-paste process that cleans up the spectacular amount of gunk-code Word will otherwise insert. To invoke it, (a) click the link for "Rich Text" at the top of the textspace for "Work" on the "Post New" page, and (b) click the leftmost button that appears on the ensuing button-bar.