Based on the discussion found on the page What is Satanism?, the simplest answer to this question would be: just by declaring oneself a Satanist!
The key quote from LaVey’s The Satanic Bible that provided this site with its name would seem to suggest much the same. For more on that passage and its significance, see the discussion here.
Yet, somehow, making the bold declaration to the world that one is a Satanist ultimately proves easier said than done. To gain an appreciation of some of the issues involved in the process, take a look at this post.