As the trophie says, you need to knock down at least one person in all of the seven saloons in the game. These are located in Armadillo, Rathskeller Fork, Thieves' Landing, Casa Madrugada, Chuparosa, Escalera and Blackwater.
The harder ones will probably be the ones on Blackwater and Thieves' Landing: in Blackwater most of the people inside are unarmed, and unarmed people most of the time will run away. You can make lawmen follow you inside the saloon by equiping the fists and "targeting" them, but in the process you will get a bounty level. In Thieves' Landing the people will directly shoot you instead of fighting if you have high honor or an outfit related to the law. You should wear either a gang outfit or the bandana.
People must be knocked inside the saloon. Out of it wont count.