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 Twinkle Help Please 

I don't quite understand the instructions for installing Twinkle on Firefox. The directions mention a monobook.js file, where can I find that and how would I edit it? Thanks for the help.  
You can find it at 39''' 
Ok, I did that, but nothing seems to have changed anywhere. Wikipedia still looks exactly the same. The Twinkle documentation page doesn't seem to provide any  information on how to actually use the script. I'm probably missing something, would you mind helping me again?"