Well, if the fanfiction is in a PDF format, it's quite easy to print. Just open the PDF with a PDF reader like Adobe Acrobat Reader and click on the print icon. But if it's just a web page, some browsers have an option to 'Print as PDF' which can be useful. You can then save the PDF and print it later. Also, check if the fanfiction is part of a fanfiction archive. Some archives have specific rules about printing, and you should follow those.
You can usually print fanfiction directly from the website where you read it. Look for a print option in the browser menu, like 'Print' or use the keyboard shortcut (Ctrl+P on Windows or Command+P on Mac). However, some websites may not support this due to copyright issues. In that case, you can copy the text (if allowed) and paste it into a word processing document and then print from there.