I don’t think there’s an actual tutorial.
Using gmail’s smtp is described in a forum post
and the wiki doc for patient self- registration in the portal
https://www.open-emr.org/wiki/index.php/HOWTO:_Patient_self-registration_in_the_Native_Patient_Portal
links to that post. But the process is not reproduced in the wiki article.
Other than that, the only instructions for SMTP setup I can find are in
https://www.open-emr.org/wiki/index.php/Sms_and_Email_Notification_Howtos
which is pretty old, as it talks about globals to set in OpenEMR v3 and 4.