On Booster Hill as you chase Booster to the top, if you touch the princess she will give you a flower point. After you reach the top of Booster Hill to Wedding Moor, return to Booster Hill and Toad will approach you about it. If you run up the hill you can retrieve flowers on your trek back up the hill. Continue doing this until Toad promps you, telling you there is no more flowers on the hill.