To be used with parsimony: A post reported simply because it contains sentiments with which you don't agree is 9 times out of 10 going to have no action taken. And reports coming regularly from the same person may lead to that person being warned rather than the poster(s) about whom the report is made.

Anyway, to do it is simple; click on the arrow next to the cog at the top right of the post and then click on Report Post, as in the image below: