All S-Video means is (1) a special connector and (2) separate luminance (Y) and chrominance (C) rather than composite video.
In a VCR, you will need to bypass the input circuitry and get to the place where Y and C are separate. This may or may not be possible depending on its design.
In a TV, they may never be separate and you will need to substitute your own circuitry for the chroma demodulator.
It is probably not worth it as you will likely not gain much in picture quality but if you really are determined, a schematic will be essential in either case.
If all you want to do is allow for an S-video input, there are single chips which will combine the Y and C into a normal composite video signal.
Also, see the section: How do I add A/V inputs or outputs to a TV which does not have them built in? since there may be safety implications in the case of adding S-Video to a TV without any A/V jacks.