You probably saw this star as you were doing the others, as it’s sitting on a large mushroom off to the side as you’re scaling the mountain. As the description of the star suggests, it wants you to use the cannon to blast to the mushroom. Before going straight for the cannon, though, you will need to speak to the Bob-Omb Buddy, which is well-hidden on this course. When you get to the point where you jump to the little area with the giant log, there is actually a lower ledge by the one connected via the bridge. With the Bob-Omb Buddy out of the way, you now need to find the cannon.
If you can’t get out of the little area with the Buddy, then just drop into the water below you. Anyway, the cannon is located near the Monty Moles and Red Coin mushrooms, on a ledge underneath that area. You could try to jump down there, but it’s not an easy jump and there’s a much easier way to get down there. Head to the Red Coin mushrooms and look for a small one smack dab in the middle. Jump to that small mushroom and stand still to be warped to the lower ledge, so follow the little trail to the cannon. From there, all you have to do is line up the shot to get to the mushroom with the star.
As you might expect, there are other ways to get to that lonely mushroom. If you get to the giant log, you can actually Long Jump off the far end (don’t move the log) and reach the mushroom with the star. Additionally, you can also use the floating from the Fly Guy near the giant log to fly down to the mushroom. This is a little safer than just Long Jumping down, by the way.