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Part of it you answered by yourself. You have to trade a shiny into RBY and then find a wild Ditto. Let it transform into your shiny and then catch it with a masterball (don't know if other balls work; haven't tried that in a while). As soon as you transfer this Ditto into GSC you should get a shiny Ditto. Since Ditto is compatible with most of the Pokemon if you use it for breeding every egg you receive has that 1/64 chance of containing a shiny. Don't really know how this tricks called but since you get a shiny Ditto as a result I might as well call it shiny Ditto trick xD
Don't try that with a Mew, though, since RBY recognizes every wild Pokemon which used transform as Ditto, so if you catch a Mew which used transform you only get a Ditto out of it.