Firstly you need to know which board version your Mega Drive II has. This mod *ONLY* works for the (uncommon) VA4 board - I'm afraid there's no easy way to add S-Video output to VA1 and other earlier boards. (The version number is printed on the board itself.)
Find a chip on the board labelled CXA1645 - this is the composite encoder, which provides S-Video as well as composite output. You simply need to make the appropriate connections to the chip, as follows:
Pin 15 - Chroma Â© output
Pin 16 - Luminance (Y) output
Pin 24 - Ground
It's as easy as that! No other components are required, you can simply wire your cable directly to this chip.