People worship Satan by performing ceremonies/rituals and praying to Satan. It's pretty much the same as worshipping any other deity; just say "Satan" instead of, say, "The Lord."
Really in my opinion I truly believe that people "Worship Satan" because they just really like to tick off Christians because if you actually look it up Satan worshipers torture humans, animals,...
Some people worshipSatan for they believe that he will give them their desires, money power etc. There are others who just worship him on a religious level they would worship him as their saviour...