First, make sure that your Ghost account is Creator or higher, Stripe is connected, and you have at least one membership tier set up (you do not need to charge to use Outpost, but do need the option set up to use all our features). Then, head to admin.outpost.pub and click Sign Up in the top right corner.
Image Credit: "Rocket Launches into Alaskan Aurora" NASA/Lee Wingfield
Go to the main documentation page. Or contact support@outpost.pub if you have questions.