Are you logged in? If so, look at the upper right of the page: In at least two places there's a link that says "Post New"
Click on it and fill in the form.
Rating: self-explanatory Warnings: the default is "Choose not to use Archive warnings," so unclick that if you have something else you want to fill in. Fandom: start typing Harry Potter and the rest of the options will appear. category: self-explanatory Relationships: that's pairings *and* friendships. Characters: self-explanatory Additional Tags: anything you want... Work title: self-explanatory Pseud: ditto Co-Authors: also ditto Summary: copy/paste what you have on your LJ Notes (at the beginning): self-explanatory Notes (at the end): I save spoilers for this category Post to Collections (start typing in hp_beholder or snarry holidays or whatever Most of the rest is self-explanatory, but... Set a different publication date: fill in the original posting date! Text: Upload *or* copy/paste from LJ