Cool beans! I'm coming at this from a Windows perspective, but this should work well in Linux or Mac as well.
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Install NodeJS. On Windows, you go to the website, run the installer, and click "Next" a few times. http://nodejs.org/
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Install stylus using NodeJS's package manager, npm, which has been automatically installed for you. Open a terminal and run:
npm install -g stylus
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Create your Stylus code and save it as something like my-styles.styl. You can probably rename your existing .css file to .styl without changing anything else.