In Sunyshore market (the grayish building) she will see if your pokemon worked "stupendously hard" and if so, will give it an effort ribbon. The pokemon has to be in the first slot in your party also.
She gives it to you once you've reached the 510 EV cap for a Pokemon. Remember each stat can max out at 255, but the Pokemon itself can't take more than 510 EVs total across all 6 stats.