It's hard to tell what you're trying to do. Are you trying to make a
composite cable out of an S-Video one?

If you're just trying to make an S-Video cable for the DC, save yourself
the trouble. You can pick up a DC-->S-Video cable by Nyko for $10. That's
cheaper than most S-Video-->S-Video cables. A composite cable should have
come with your Dreamcast.

If you still insist on building your own, pinouts can be found here:
http://mc.pp.se/dc/vgabox.html

-Gary