Visit https://infosec.exchange, and click the Create Account button.
A page will then display the Infosec.Exchange Rules. Click Accept to confirm that you understand and accept the rules (or click Back if you don't, but you won't be able to create an account if so).
On the next screen, enter your:
Click SIGN UP and you should soon receive an email with subject:
Mastodon: Confirmation instructions for infosec.exchange
It might not appear instantly if the server is very busy, but check your spam folder if you don't see it within a few minutes.
Click the Verify email address button in the email, and you should be good to log in!