Answer:
Simply by deciding if you agree with the church of satans ideas or not. If you agree and feel it is right for you then you are a satanist.
If you want an official membership and a crimson card however, you have to pay 200 dollars (last time I checked, but check out the Official Church if Satan website anyways for more info) to the church of Satan and they will register you and send you a crimson card. You may have to write a bit on why you want to be a satanist.