depending on what the dvd player/cable box/tivo has on it, you can plug it into the "INPUT" or "A/V IN" or "Video IN" plugs on them.
try plugging it into the dvd player input (if it has one)
set the tv to dvd player
set the dvd player to "input device"
every dvd player, tv, cable box is different. so there's probably more to it than to find the input or video button on the remote.
they also make switches that you can push a button and switch back and forth. $20 US