First, you need an iOS Dev Center account that has the role Agent or Admin to distribute apps.
1. Logon to the iOS Dev Center (here).
2. Go in the iOS Provisioning Portal.
3. Go to Certificates section and Distribution tab. Make sure you have a valid certificate. If you don’t, follow this guide to get one, then come back here.
4. Go to the App IDs section and add a new App ID for the app you want to distribute. Note: It is one App ID per app, so if you already created one for your app, skip this step.