As the guide says above you need 20 banners in the game, if you would like help to get this achievement I have a save file which has over 27+ banners.
If you want help for the achievement message any of the gamer tags below. or leave a message in the comments section.
Also the achievements are NOT STACKABLE. If you join a map with over 20 banners you will recieve the 20 banner achievement, but you will not recieve the 10 banner achievement. If this does happen do not worry just build a game and allow 10 different people to build their banners.