All you need to do is set up a customer account (not a publisher account) through any one of OneBookShelf’s sites (i.e. DriveThruRPG, RPGNow, etc.). If you already have an account on any of those sites, your existing account works.
Next, go to the Account page at DriveThruRPG.com (or on DMsGuild.com for the D&D program) and look for the My Content section of the page. There you will find links to webpage tools that allow you to upload your new title, edit your existing titles, and see sales reports for your titles. The My Money section of the Account page also has a webpage tool that allows you to withdraw royalties via PayPal. Note that the My Money section may not show on your Account page until you have a royalty balance from sales.