To access .STUDIO, simply click on the instance link provided to you, typically formatted as "companyname.poweredby.studio." Once on the login page, enter your email address and password.
If you're logging in as an admin for the first time, your customer success manager will provide you with the instance link, user id(email id), and password. Upon logging in, make sure to change your password through the settings page.
For standard users, an invitation containing the login link will be sent to you. Follow the link to log in and set up your password.
If you ever forget your password, utilize the "forgot password" link for assistance.