First, you need to own a Gallade. Then, hold up to 6 Dusk Stones in your possession at any given time and make sure that none of them are currently being used as an item or held by someone else. Once the process has finished, go into one of the menus on your game and access the Evolution screen for Ralts (or if it is already evolved then choose Gardevoir). Here you can scroll through all Pokemon with Dusks in their name until you find what youre looking for.