This came up in the Core editor channel:
https://wordpress.slack.com/archives/C02QB2JS7/p1612864728329100

We need easy instructions on how to test a PR directly in the PR template.
Making it easier for anyone to download a special version of the Gutenberg plugin that includes the PR.

It would be helpful to add these instructions to the PR template.
One method to test the PR is to download a special version of the Gutenberg plugin.

1. To the right of the Conversation tab: Click the Checks tab.
2. In the left you menu bar (scroll): Click Build Gutenberg Plugin zip.
3. To download: At the bottom: Click gutenberg-plugin
4. Unzip the Gutenberg plugin. Inside is another gutenberg zip containing the plugin. --> This is the zip that needs to uploaded to the plugin screen.

I downloaded the zip file, and double clicked to unzip. The Mac will automatically unzip the zip and the zip inside it.
Right clicked the unzipped file and compress it to create a new zip. Then upload that...

@gziolo @noisysocks @annezazu