1. Login to the site with a user that has permissions to edit the form.
2. Visit the form you want to edit, and click the Webform tab.
3. Underneath that tab is the Emails tab. An example URL of where you'll ultimately land is https://www.example.com/node/12345/webform/emails
4. Choose "Email" as the Component value, and click the Add button.
5. Fill in the resulting form, and that will be the email will be sent to people who fill out the form.

If you are sending these emails from an email address with your custom domain, you will need to configure Platform Email. If you use a 3rd-party email service, contact that service for the SPF records to add (and anything else you need to confirm your service as a valid sender of email).

We have documentation on troubleshooting email issues with Drupal but please let Acquia Support know if you run into difficulties.