Alright, here we go
First, we need 2 different consoles with one having RBY and one having GSC. You need a Pokémon with shiny IVs (these determined shininess in these games) and trade it down to RBY. It maintains these shiny EVs. People typically use Gyarados for this, as it’s a free shiny that works perfectly fine.
Now, go to Celadon City Mart and buy a Pokedoll. It’s on the fourth floor.
Head to Saffron. You’ll find a girl who will give you the TM mimic for the pokedoll. Teach your shiny Pokémon Mimic.
Now, find a ditto. It’s locations are at Route 13, 14, 15, 23, and Cerulean Cave. Make sure your Pokémon is faster than the ditto. First, use mimic, and let Ditto transform in to you. You mimicked Transform, so wait for it to use Transform again. Now, it has your shiny stats. Capture it, and if it uses Transform again, that’s fine, it’ll still be shiny.
Now, trade the ditto to GSC. You have yourself a blue ditto!
Sources:Personal experience and this.