I still don’t understand what you are trying to do. Why do you need GitHub, and why do you download the Ruby file?
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You copy the code that was shared in the other topic:
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You open the browser console using, for example, F12 (or another way to open the console. A search engine can help you find guides on how to do that in your browser).
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You paste the code you have copied into the console:
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You execute the code by pressing Enter.