A universal remote is a remote that can substitute for the remote that came with your TV, once it's been programmed. While it's recommended you have the TV active when programming is recommended, you can still do it while the TV is off.
Press and hold the "Program" button on your remote. Note that this button may also be titled "Prog" or "Setup." Wait for a signal that your remote is ready to be programmed, such as a light glowing solidly or buttons flashing. (The specific signal can be found in your remote manual.)
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Choose the "TV" option on your remote. If there is no button, or you do not see a choice between devices such as Cable, VCR or DVD, skip this step.
Enter the code in your remote's manual that correlates to your TV using the remote's number pad.
Press the "Program" button again to finish programming the remote.