*First of all, make sure to save states, so you don't lose any progress.
*Enter the Debug (On the Silver/Gold Screen Version press Select)
*Press A on the second option, the character will shrink and start the game on it's room.
*Catch any Pokémon from Kanto (you have x30 Balls on your bag), leave the Johto Pokémon on the PC, you can find a PC on the south-west house in the first town. (You can also find 3 random Pokémon inside the PC sometimes)
*Go near the GATE where Silver is and the Demo ends.
*Press B to walk through walls and run, go to the doors of the GATE without facing Silver.
*Go through the GATE, now you're on the second town, the Pokémon Center is on the East.
*Inside the Pokémon Center, go upstairs (on the left side).
*Start Link with your emulator(s) on Pokémon Red/Blue/Green on Japanese, doesn't work with English versions.
*Press B to walk through the door and access the Time Machine, then go where the Nurse is.
*Try to start the Trade in the Time Machine until it works.
Trade is a little buggy sometimes and it takes some tries to make it work properly, also some Pokémon traded can crash Red/Blue/Green, but don't be scared, this can be fixed. (I recently traded a Shiny Vulpix which crashed everytime I looked into her Stats, but this was "Legality problems" easily fixed with PkHex)