Gmail: Sending Email from Gmail as a Google Domains Email

This gave me so much trouble, and had to dig way down to find it, even though I already have done it before, so I'm putting it here for future reference.

1. Turn on 2-step verification if you haven't done so already.
2. Goto your Account -> Signing In -> App Passwords
3. In the App passwords box, select Mail for the app, select Other for the device.
4. Enter the name of your domain for the “Other” device, and click Generate.
5. Goto Gmail settings, Accounts and Import tab.
6. Click on Add another email address you own
7. Type name and email address to be added from a domain hosted on Google Domains. (It didn't seem to matter if treat as an alias was checked or not).
8. For SMTP Server, put smtp.gmail.com
9. For Username, your full Gmail address including @gmail.com
10. For password, your App password generated from step 4.
11. Leave the rest default (TLS, port 587)
12. Click Add Account

For more details see Google Support - Sending mail from your forwarded email address.