I Have Studied or Worked Already at TUM. Do I Have to Follow Certain Proceedures Becoming a Guest?

No, there is nothing special to do. If you had a central account at TUM after about 2004, your old account is still active, but has only alumni status. The account will get a new status/profile as soon as it receives the guest status.

If you never had a central account, you will get a new one.