you haf to beat the eliet four and hav the national dex, so, u go to canalave city, from the pokecenter, u go south, then ull see a house next to a sign, itl say, "sailer ... house" u go in, first talk to the boy sick in bed, he'll say somthing about the pokemon Darkari, a legendary pokemon (a nintendo event, but thats a different story!) then, talk to the little boy's mother, (radda radda radda) then go out side and speak to the sailor, he'll say somthing about a cresent moon, then, he'll offer u to take to "New moon island" say yes, watch the ship go and leave, then u enter a patch of flowers leading a path. follow it. and ull see a clearing, and in the clearing, ull see a pink and yellow thing, talk to it, it appears to be a pokemon! It is a Cressilia, it will be in ur pokedex, then fly away, its just like catching Mespirit.

NOTE!!!!
it'l be a lot easier if u haf the poke app, poketraker, itl haf a bear face, so thats were cressilia is! to git the app u need to fly, (or walk... if u haf the paitence) to Jubilife city and go to the poketech compony. ull see a fat guy, hes the president. note: hes on the first floor next to the counter. he'll give u lots of apps. one of them is the poketracker. now, id git a Wobbafet to catch it, i had a lv 61!

Chow!
hoped this helped!