Manually. You print the confirmation letter and then e-mail or mail to the client. 

wow, I wish Lacerte sent automatic emails. This should be in the program to do. Thank you for your response

Vote for it here https://proconnect.intuit.com/community/lacerte-tax-idea-exchange/automate-client-notification-of-ir...

This is how changes sometimes get incorporated into the program from user wishes.