Why S-Video? If you don't have a setup for SCART or RGB then this is the best video quality you can get for your retro systems...and it is DAMN good looking. The jump in quality from composite to S-Video is dramatic, and the jump from coaxial (if somehow you're stuck with that) to S-Video will flatten your balls.
Alternatives to this mod: You can find an S-Video cable for Saturn easily on eBay or Amazon for about 10 bucks, making this mod...kind of pointless.
HOWEVER, if you find modding your consoles fun (as I do), you want to use easy-to-find generic cords instead of relying upon proprietary Saturn cables, or you're worried about the Saturn's default video output port failing (It can get finicky over time) then you should give this a try. Having separate stereo sound ports also makes it easier for you to hook your console up to external speakers or for capturing sound and video from the Saturn.
There are a lot of photos here so click below to view the tutorial.