You’ve gotten all of the stars outside of the volcano, so it’ll finally be time to head inside the volcano. The path with the pyramid-shaped platforms (right at the initial fork) will be the easiest way to get to the volcano, so jump inside once you get there. Upon reaching the inside of the volcano, you will see two paths before you, one of which leads to an elevator and the other is a normal path. You will want to take the path that leads to the elevator, which is a few jumps on some small platforms. The first elevator is an uneventful ride, but it will fall off the track at the end, so make sure you jump onto the second elevator.
The second elevator is a shorter ride, so when you see a pole in view, make sure you grab onto it. Climb to the top of the pole and wait, as there will be a Fire Bar rotating on the next platform and if you hit the Fire Bar, you will likely lose control enough to run off the edge, possibly into the lava below you. As soon as you land on the platform with the Fire Bar, quickly jump and grab the next pole. At the top of this one, you will have a few more tiny platforms to traverse before reaching the star.