Ok, look at your cable box. It will have some form of video out on it. It probably has an coxial cable (screw on) connector, and/or an RCA style (usually colored yellow), and/or an S-Video output. Choose one of the above, and run the appropriate cable from the VIDEO OUT on the cable box to the matching VIDEO IN connector on your projector. Of the three mentioned above, S-Video would give the best picture quality.
Voila.
I'm not sure why you're hooking the projector up to the TV. That doesn't make sense.