First, search for and download the CBS app. Then:
1. Launch the app
2. Select Settings > Subscribe.
3. Select the Commercial Free subscription plan.
4. Select "Using your email", then enter your email and complete your profile information.
5. Confirm your subscription. 

You'll be returned to the Settings screen. From there, simply select an option from the menu (like Home, Shows, Live TV, or Movies), and start streaming CBS All Access commercial free!