Downloading and installing

Userscript version (recommended)

Make sure you have the Code Injector userscript installed, then download the gmmaker-(version)-USERSCRIPT.zip file from Releases and, depending on the userscript extension you're using:

In Tampermonkey

Open the Tampermonkey dashboard.
Go to the Utilities tab.
Click the "Choose file" button next to "Import from file", choose the zip you downloaded, and then, when the installation tab appears, click the "Import" button.
Reload Bonk.io.

In Violentmonkey

Open the Violentmonkey dashboard.
Go to Settings.
In the Backup section, click the "Import from zip" button and choose the zip you downloaded.
Reload Bonk.io.

Extension version

Download the gmmaker-(version)-EXTENSION.zip file from Releases and then, depending on your browser:

In Chrome (and other Chromium-based browsers, hopefully)

Go to chrome://extensions/
Enable Developer mode in the top-right corner of the page.
Drag and drop the zip file into the page.
Reload Bonk.io.

In Firefox

Note: You will have to do this every time you restart the browser.

Go to about:debugging#/runtime/this-firefox
Click Load temporary addon and open the zip file.
Reload Bonk.io.