Once you've completed your story and are ready to submit it, just click the "Submit for Review" button at the top of your draft.
If you don't see the Submit for Review button yet, it's probably because you're missing some required content. All stories need to have a title, a subtitle, a featured image or video, and some text content before you're able to submit. Once you have those required fields filled out and have saved your story, the Submit for Review button will become available to you.