In the old days they used to cover the drywall with plastic moisture barrier of felt paper. This is the way I used to do it.
The plastic/felt goes over the edge of the shower pan. You can staple it.
Then attach CBU per instructions. You can red guard it or at least the bottom 2 feet. You are now ready to tile.
You can visit johnbridgetileforum for more ideas, information and details.