I think you are thinking of the cloning glitch in gold and silver. Here's how to clone in pokemon red.
**THIS GLITCH CAN DELETE YOUR GAME SAVE DATA.**
**This glitch requires 2 Gameboys and a link cable.**
Have one Pokemon game with a pokemon that you both want (generally a starting pokemon, a legendary, or Mewtwo). Have the other game have a pokemon that you wouldn't miss, as it will be deleted forever (Example: Level 4 Ratatta). Once you both have your pokemon, enter a trade. Now, trade the bad pokemon for the good pokemon, and at the end of the trade, right after the screen flashes "Waiting..." but before it flashes "Trade Completed," the person who originally owned the good pokemon should turn off their Gameboy or pull out the link cable (turning off the power is the preferred method, but should work). The person who originally had the bad Pokemon should have the message "Trade Completed" on their screen.
If done correctly, you will both now have the good pokemon. The person who originally had it turned off their Gameboy before it could save, so his gamepak doesn't know the trade ever took place. The second person's Gameboy remembers the trade, and has the new Pokemon.