Instructions
Start any level in the game.
Play through the level, taking extra care to cover every nook you come across. Some walls have hidden tunnels, and some pipes take you to extra parts underground. Some coins are cleverly placed in these secret areas and are frequently missed.
Walk into the star coin to collect it. Star coins are considerably larger than normal coins, so it will not be difficult to recognize them. Collecting the coin places it in one of the three coin slots on the top corner of your screen. If the first coin you collect appears in the second slot, you missed a coin along the way. If it's in the third slot, you missed two coins. If it's in the first slot, you found the first coin and have two more to go.
Finish the level to permanently keep the star coins from that level.
Replay levels to find star coins you missed. Star coins you have collected do not need to be re-collected.