you need to meet Sonia on the outskirts of Freezingtown unnder a tree, then after the cutscene will trigger a event wich you will have to find evidence footprints around the wild area, it looks almost like a sparkling item but you can see is a footprint, they are kinda easy to find, there are 150 footprint total, 50 for each pokemon, each footprint count as 2%, remember there are 3 set of 50 diferent footprint names the progress will count separately, in one of the freezing town houses you can meet sonia again talk to her to check the footprint progress, when you find 50 footprints 100% of one pokemon Sonia will tel you the location of the pokemon, legendary pokemon will spawn in the wild like the roaming pokemon, they are easy to find.