Put the Sweet Honey on the Honey Tree. Return back to the Honey Tree in about 24hrs, and the tree will be shaking. Interact with it, and it might be Cherubi, or it might be something else. Keep on trying until you get one.
Catch it, take it to Level 25 and voila! Congratz, you have Cherrim! :D
In order to get the other form of Cherrim though (the blossomed flower) then make Cherrim play in-battle with sunny weather. Apparently, at Level 25 it learns a move which makes the battle field sunny, which is good. Then, you have a blossomed Cherrim. :)
Blossomed Cherrim can only be available in battle however. Out of battle it is unblossomed Cherrim. :)