Instructions
Connect two Game Boy devices using the Game Boy link cables. One of the Game Boys must have the "Pokemon Silver" cartridge inserted while the other Game Boy can have any cartridge as long as Mewtwo is saved in the cartridge.
Power on both Game Boy devices. In the game, both players will need to go to the nearest Pokemon Center and head to the top floor using the stairs in the bottom-left corner of the Pokemon Center. Both players will also need to speak with the receptionist on the second floor in the Trade Center at the same time.
Wait for the receptionist to explain the trading process. She will ask you to save the game and move towards the trade table.
Transfer Mewtwo from the other Game Boy device to the Game Boy device with the Pokemon Silver cartridge. At the trade table, press "A" in the other Game Boy device to select the Mewtwo to be transferred. A message will appear asking if you are sure about the transfer. Select "Trade" to finalize the trade.
Double-check if the transfer was successful by checking your current Pokemon party. If your current party is full, check Bill's PC in the Pokemon Center to see if Mewtwo has been sent there.