Pinlet is a Safari extension for the bookmarking service Pinboard.

I don't have a developer account with Apple, so I wasn't able to build this extension properly¹. That being said, the process is still somewhat straightforward, especially for someone who's savvy enough to look for Pinboard extensions on GitHub. Here's the step by step guide:

1. Clone the repository or download it as .zip
2. Enable the Develop Menu in Safari
3. From the menu, click on Show Extension Builder
4. Click on +, then Add Extension...
5. Locate the folder of the repository, then click on Select
6. Click on "Install" and enter your password
7. Once you've installed the extension, go to the Extensions tab in Preferences and enter your Pinboard API token, which can be found here

Note: You'll need to reload your open tabs to make the extension work.

¹ This also means you have to reinstall the extension every time you relaunch Safari. This is a non-issue for me but I recognize that it'd be a deal breaker for most people. Unfortunately, I can't fix this unless I pay Apple $100 annually.

GitHub users ArthurHammer & tsbehlman have been extremely helpful. My thanks to them, as well as other random strangers on the internet whose code I've used.