Log into your ISPConfig instance
Log in with your username and password
Click on the Email icon on the top bar
Select Email Mailbox from left menu
Click on Add new Mailbox button (green button)
At a minimum:
Fill in Email (the part of e-mail address before the @)
if you have more than one domain, select the domain name
Enter the password twice
Optional
You can also turn on/off the spam filtering. If you do that, you can also go to Mail Filter tab and tell it to move spam to the Junk directory
Click the Save button (green button, bottom right)
Test your e-mail account (may take up to 5 minutes for it to populate)
Log into webmail (RoundCube)
Use your full e-mail address for the username
Log out.