Update: appears to have stopped working since Chrome 80
Drag & Drop the '.crx' file on to the 'Extensions' page
Settings - icon > Tools > Extensions
( the 'hamburger' icon in the top-right corner )Enable Developer Mode ( toggle button in top-right corner )
Drag and drop the '.crx' extension file onto the Extensions page from step 1
( crx file should likely be in your Downloads directory )Install
Source: Chrome YouTube Downloader - install instructions