You've done it - you've written your story for a challenge on the AO3! \0/ Congratulations. Now you just have to post it, and hopefully that will be easier than the writing was...
The easiest way to post your work is to first go to the collection it's going to be part of. Be sure you get the right one! It's worth bearing in mind that many challenges which take place on a regular basis have subcollections for each specific challenge, in which case you need to be sure you go to the right subcollection. The fastest way to find a challenge you've signed up for is via the "My Signups" page, which is linked in the sidebar of your "My Home" page.
In the example below, we're going to be posting a work to the "Awesome Exchange 2010" subcollection, which is the 2010 round of the "Awesome Annual Challenge" collection.
Clicking on the "Post To Collection" link will take you to the form for posting a new work, which you can fill out as normal. In the "Associations" section, you need to make sure that the name of the collection or subcollection you are posting to is listed in the "Post to Collections/Challenges" field. If you've come to the form by clicking the link on the collection's page, this will be added for you automatically.
If you are participating in a gift exchange, you should enter the name of the person you've written your work for in the field immediately below that. The form will suggest possible matching names to you.
Once you've finished filling out the form, you can preview and then post! \0/ Bear in mind that if the challenge is set to be anonymous or unrevealed, your work may only be fully visible to you. For example, you may see something like this:
If your work is currently anonymous, however, other people won't be able to see your name after Anonymous, and if it is currently unrevealed, other people will see only a "Mystery Work":
If you're nervous about whether this is really working correctly, one thing you can try is logging out and seeing how your work appears to you then - you should also then see the Mystery Work blurb or the anonymous blurb. The owner or moderator of the challenge will reveal your work when the time comes!
Hopefully this should help to ensure that the process of posting your challenge fic goes smoothly, but if you run into any problems, please contact the Support team and we'll do our best to help!