1. Select text on a web page using the mouse.
2. Press a keyboard shortcut to search for the selected text on your desired website.
Alt+Q = Google
Alt+W = Wikipedia
Alt+A = Google Images
Alt+S = YouTube
The target search websites and keyboard shortcuts can be changed from the extension settings menu.
The permission to "Read and change all your data on the websites you visit" sounds unfortunately scary. It is required for the extension to have access to selected text when a search is performed.