Propaganda is in the form of posters found on walls throughout the game. The posters are red. You must touch it with your character and it will rip apart. To check your status in each level, hit and then for statistics. You will see how many you need for that mission. Also, you can go to load from the start menu, go to select mission, and it will show a list of your progress. There are 36 posters in the game, meaning more than needed for the achievement. If you miss a few, you will not have to worry. Each one also gives you XP.
For locations on these posters, click HERE.