Installing Chrome extensions manually from GitHub is a great way to pass around extensions if they are a work-in-progress or otherwise don't need to be in the Chrome store.
Here are the three steps to make it happen.
Use either technique to get the folder onto your local machine. Use whichever technique fits your workflow and comfort level.
If you clone or fork the repo, you'll only have to
git pull every time you want changes. You will still have to manually "refresh" the extension to see the changes, explained later.
If you pull down new changes or make a change to the extension's files locally, you'll need to hit the "refresh" icon in
chrome://extensions/ in order to run the new functionality...
You do not have to understand how to develop Chrome extensions in order to download and use one, but if you are interested in getting started, here is a nice post...
For more info on working with Chrome extensions in general, search DEV for "chrome extension" and find a lot of great resources. 😄