How to use Simple Notes for Gmail
(aka Simple Gmail Notes) in Safari
1. Install extension from Safari Extension Store
2. Open the newly installed app, and click the button to open Safari preferences:
3. The extension should be shown in the list, check the checkbox to enable the extension
4. Log into your Gmail account, and click the icon near the address bar, click 'Always Allow on This Website'
5. Click into any email, and you could start writing notes there: