Click the green Accept button or red Decline button. A dialog box will appear where you can select a Response Template to populate the email message that will be sent to the submitter, or simply paste in the email text. Read more about Response Templates here.
Optionally attach a file to the email and choose the email address that you would like the email message to be from. Add an internal note if you'd like and then click the button to change the status of the submission to Accepted or Declined and send the email.