Unzip it. You'll get a grammar-ai-extension folder.
Open
(click to copy, then paste in a new tab)
Turn on Developer mode (top right), click Load unpacked and choose the folder.
Pin it from the puzzle-piece menu, then click into any text box.
Why the extra steps? The extension isn't in the Chrome Web Store yet, so browsers need it loaded manually. It only talks to this site's server and never reads pages you haven't typed in.