Enabling 2FA
Go to your global Profile page (not a specific community) and click Enable 2FA under Two-Factor Authentication. Scan the QR code with an authenticator app and confirm with a 6-digit code.
Backup Codes
Once enabled, click Manage backup codes to generate a set of one-time recovery codes. Use one instead of your authenticator code if you lose access to your device — each code works once, then is spent.
Disabling 2FA
Enter a current 6-digit code (or an unused backup code) next to Disable 2FA to turn it off.