you can't. dubious disc is to evolve porygon 2 into porygon z, porygon z is a sinnoh Pokemon and leaf green only goes up to hoeen
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A dubious disc is a hold item for Porygon2. If you trade a Porygon2 that's holding a dubious disc, it will evolve into Porygon-Z.
The Pokemon that uses the Dubious disk is Porygon2 to evolve him into a porygon-Z
You find the Dubius Disc in the P2-Labratory in pokemon white. Hope that helped :)
The Dubious Disc, when attached to either a Porygon2 or a Porygon-Z and traded, evolves the Pokemon. In HeartGold, it's found in Johto on Route 42 (requires Rock Climb).
If you give the item to a Pokemon to hold, and then trade it, then your Pokemon will evolve. I am not sure if this only applies to Porygon or if it will work for any Pokemon but I hoped this helped