After you're experiment has passed QA and you've mitigated the risks, you are ready to launch!
You're experiment should be in Preview mode - from the QA testing and/or your self-test.
At this point you check off the questions accepting responsibility and verifying that you've completed the training, then click "Request Launch"
Now you follow the directions in Experimenter: Copy the URL for your experiment. Include the URL and ask for review to launch either from a reviwer on your team or go to #ask-experimenter and ask for someone to help review and approve the launch. If none of the general reviewers approves the launch it in a couple of hours - ping again. It is usually quite quick.