In general you use Ruby code like this
kanji <ruby>kanji<rp>(</rp><rt>furigana</rt><rp>)</rp></ruby>
Then there are also two scripts to make it easier
- [Userscript] Forum: IME2Furigana
This one works with your IME to add the proper furigana
- [Userscript] Easy Furigana
This one lets you put whatever you want as the kanji and furigana, and lets you convert multiple instances at once

