Despite what the other guide says regarding 'mastering' a level, in order to master a level you not only have to get all three stars and any treasure, but you also need to collect all coins in the main level as well, (so that doesn't include coins in the bonus stages, but you will still need to get the treasure boxes in those stages on a few of the levels). It's easy to tell which levels you've mastered when viewing the map, as they will then say "100% complete" and the frame showing your best time and stars will change to a fancier gold border. There is a very comprehensive guide on Steam which tells you where all the coins are on every level.