The igloo is the indoor section of this course, and getting to it usually requires you to use the Koopa shell located by the freezing pond. Take the warp to the freezing pond and use the Spindrift to get to the upper ledge, where you will break the box on the right to get the Koopa shell. Ride it back down to the freezing pond and locate the steep slope with the coin trail. Go up the slope and you will run into a fence, which is where the igloo entrance is located. Note that if you were going up the path behind the source of the moving mounds of snow, you can Triple Jump over the big wall there to get right to the igloo, which is nice in case you lost the Koopa shell.
Once you’re inside the igloo, you will be able to see the star encased in ice straight ahead. Unfortunately, you can get to the star normally, but there is a Vanish Cap block in the corner opposite the entrance. Jump over the ice wall, don the Vanish Cap and double back to the star.