First, download and launch the CBS app. Then:

1. From the menu, select Settings > Subscribe. 
2. Select subscription plan (either Commercial Free or Limited Commercials). 
3. Select "Using your Google account". (If you don't have a account and/or a Google Play account, select "Using your email" and follow the instructions.)
4. Complete your user profile and accept Terms of Use.
5. Select "Subscribe".

That's it! Now you can start streaming CBS All Access.