The way to have the best chances really is just to trade with a lot of different people.
GTS can get you a lot of different trainer ids which will give you the best chance of matching one of the ids with the drawn number.
When you walk it there is a woman at the desk and she'll ask you if you want the your "lotto" ticket say yes and you'll figure out the rest when you talk to her
you have to go to the trainer school which is really close to the entrance and when the brown headed guy starts blabering on about poketchs he sends you to find three clowns and then you have to...