Put Ruby in the GBA slot of your DS. It's on the front. Turn on your DS. Select Pokemon Platinum Version. Scroll down on the Platinum menu until you see "Migrate from Ruby". Select that. Then choose the Poke'mon you want to migrate by tapping them. They cannot go back to Ruby, just warning you. You can only do 6 per 24 hours, and you have to catch all 6 before migrating again. Poke'mon that know HMs cannot migrate.