I'm not sure that's easy (I saw in another post that you're in high school). It is complex enough to require some dedicated ICs. If you're still interested, look up 'video decoder' ICs and video DACs. They are all dense surface-mount parts, and it would be hard for you to design it from scratch if you're not used to interpreting datasheets - plus it is high-speed design. It used to be easier when everything was analog, but most (all) of the ICs which supported that are now obsolete.
