You download the app, click sign up and create your One Coworking account. In order to use the service, you will need Roaming Credits. You can get credits by either buying a plan or connecting your existing coworking membership.
Here is a guide on how to connect your coworking membership if you're using iOS. On an Android device you simply go to menu, choose 'Link Membership' option and insert your existing coworking membership credentials.