This is where it all starts: your TV-out. Some cards have a composite video out, but most cards only have a S-Video out, and a very small group have both.
Left=S-video, with the Composite next to it (normally yellow)
Before you can start, you'll have to check if you even have a TV-out at all, because not all cards have it. I've seen TV-out's on just about any card, it's a matter of having a card from the right manufacturer.
ATI card are known to have a superior TV-out compared to nVidia and Matrox. I don't know if this is true, but i'm not going to stare half an hour at a TV to see the difference, because it's minimal. The loss that you'll get by transferring the the signal trough all those cables is bigger then the loss between different chips, so basically you won't notice.
Ow wait, my TV-out doesn't look like that at all!
That's possible, some manufacturers like MSI and Abit use other connections similar to S-Video because they have more options.
For example: My MSI GeForce 4 Ti4400.
Doesn't look like S-Video at all. But all those manufacturers include some cable to change that connection to a regular S-Video and/or composite TV-out.
MSI: S-video IN, Composite IN, S-Video OUT, Composite OUT. Not bad huh ;)
From the Abit Siluro cards: S-Video and Composite