You must find all flags in the Poor, Middle, and Rich districts of Jerusalem.
for an awesome guide to the Jerusalem flags (as well as all other flags) and the Templars.
Please give full credit to FiReStOrM^
for his awesome work and dedication to making this incredible resource.
In case his page goes down again,I have left videos of all Jerusalem flags in all districts. Credit goes to youtube user TheGameStory
Videos 1-2 are Rich District
Videos 3-4 are Middle District
Videos 5-6 are Poor District