Part 1 (expiration reminder)

1. go to Admin Console
2. go to Configuration → Users → General section
3. check Advanced User Management checkbox and save changes
4. go to User Management → Users section
5. create a user
6. open that user for editing
7. go to Groups tab
8. use Add toolbar button to add user to the Subscribers group
9. from group list open membership record, that was just created for editing
10. set membership expiration to 5 days from now (the period in days should be smaller then value of Group Membership Expiration Reminder (days) setting in Configuration → Users → General section)
11. save changes to user record
12. run the in-portal scheduled-task:run membership_expiration command from command line
13. go to Email Queue tab after switching to the User Management → Mailings section
14. click on Process Queue toolbar button
15. confirm, that both e-mails were sent out and all info is present in there

Part 2 (expiration)

1. repeat all of above steps, but this time enter membership expiration date in past