Upon starting the course, you will be presented with two paths, a right one and a left one. For this star, you will want to take the left one, as it’s more direct in getting to your destination. On the left path, all you have to do is perform a Long Jump over the gap to get to the door. The next area is called the Black Hole, which will have a massive hole in the center of it that leads to taking one of Mario’s lives. For now, just follow the red metal grating path and ignore that wooden door you pass along the way.
At the end of the path, you will want to Long Jump to the green path, where you might see some boulders rolling by on it. Be mindful that the boulders will not be rolling in a straight line the entire time, but sticking to the left wall should let you avoid most of them. At the top of the incline, enter through the next door to see a colorful platform in front of you, which is an elevator. Step on it to have the elevator take you all the way down to the Underground Lake. Follow the path down here to the water, where you should spot an island in the center of it with the star.
The problem is that you won’t be able to jump to it from the water, so you will have to find another way onto the island. Take a look around and the way should become clear, as there is a dinosaur-like creature swimming in this underground lake (and it’s also the name of the star). You can get on the creature’s, whose name is Dorrie, back, so do that now. Performing a Ground Pound while on its back will make Dorrie lower its head, allowing you to run and stand on it. While standing on its head, you can face a direction to have Dorrie turn that way. With this in mind, guide Dorrie to the central island so you can jump on it and grab the star.