Star 1: The Red Coins Edit
It's the first you will see on your way to the final star. After going through the level you will encounter 2 mushrooms that fall over, a quick long jump to the middle of the 2nd one grants you the first red coin. The others are spread out throughout the small section after the 1st coin. One is hidden on a small bush, another is on the Chain Chomp's pole, another is on the highest point of the section, and the others are easily visible. The star spawns before the slide.
Star 2: The Star on the Mushroom Edit
Once you go on a slide section shortly after the red coins, there is a star on a mushroom behind the third curve. Time your jump to land on the log and get the star.
Star 3: Complete 1-2 Edit
Finish the slide (after star 2) to get the last star.