What steps are needed to cancel one's account and annonymize the existing content?
You can get your account deleted by asking the moderators in chat for example, or you can set your user name as "please delete me".
For example try typing "please delete me" here.
You can change your name and, if you really want to, ask the team (email@example.com) to delete your account. It is however close to impossible to entirely anonymize your participation since that would require to modify/delete @replies by other users as well. It would be a shame to loose you as a valuable participant though!