First go here.
Click on "Submit an article." Edit: As Denise pointed out in the comments, you could also click on the GIANT ORANGE BUTTON in the top right corner. That would work too.
Fill out the form, submit it, and you're all done. Make sure you submit the permanent link to the exact post you want to feature, not just your home page.
They don't have to be brand new posts. However, please only submit your own posts. If you see one you like, comment to the author and tell them to submit it. I have no idea how many submissions we'll get so you may or may not be in the first carnival. This is the first one, so spread the word. We have no idea what we're doing so we may need to clarify some rules later. I'll update this post as needed.