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It'll be in the All Apps area of the Start menu in Windows, and in the Applications folder on macOS. The uBlock Origin extension will also block most other ads on the web. This opens the Chrome web store. A list of search results will appear. The developer is ″Raymond Hill,″ so make sure you select the right extension. You may also have to provide the extension permission to install—if asked to do so, click Add extension to continue. This installs uBlock Origin to Chrome. Press Ctrl+T on Windows or ⌘ Command+T on macOS. Now that you've opened Gmail in a new tab, the uBlock extension will block ads that would normally appear in the Promotions tab. Since uBlock Origin blocks ads on most websites, you may have trouble seeing certain content. If you can't follow a certain link or use one of your favorite sites, click the uBlock Origin icon (a red shield that says "uo" inside) in the browser's toolbar, then click the large blue power button icon to disable uBlock for the current site.

Summary:
Open Chrome on your computer. Navigate to https://chrome.google.com/webstore. Type ublock into the search bar and press ↵ Enter or ⏎ Return. Click uBlock Origin. Click Add to Chrome. Open a new browser tab. Open https://www.gmail.com in the new tab.