This level is dominated by cobwebs, which can be climbed on but temporarily disappear after leaping off of them, and Scuttle Bugs, enemies that hang from above waiting to strike. Start by jumping onto the nearby cobweb and hitting the block above to get a Super Leaf. When you reach the ledge, wait for the Bug to drop before jumping over to the other side. Bugs are vulnerable to being jumped upon, so feel free to dispatch of it. Use the cobweb to continue jumping onwards. Eventually, you will see a ! switch. Hitting it allows you to access the yellow pipe to the left. Inside the cave, hit the ! switch hidden in the middle brick to open up a path to the left. Hurry before the path disappears and jump on and off of the Scuttle Bug at the end to get the first star coin of the level. Be sure to be quick, as it is possible to end up dying by falling into the acid below! When you are done, exit via the yellow pipe on the right.
Once you are out, you will see a number of scuttle bugs on your right. To get across, you will have to bounce off of them while they're hanging low. Alternatively, you can use a Super Leaf to glide over them. On the other side is the midpoint flag. After reaching it, go up to reach the red ring of this level. It can be moderately tricky to get all the red coins in time, especially since the cobwebs disappear, but there is no real penalty for failing. Further on, jump on the second Scuttle Bug you see to reach a yellow pipe. This takes you to a secret area, where the second star coin is. After hitting the ! switch, you will have to jump up the new stairs and then hop onto the Scuttle Bug at its high point to get the coin. Once done, exit the way you came.
The final star coin is a short distance ahead. Carefully navigate the lower cobwebs, preferably with a Super Leaf, in order to grab it. Safety is key, so feel free to miss it and land on the opposite cobweb rather than landing in acid. From there, jump along the path of cobwebs leading up and right to reach a scuttle bug. Jumping on it when its high enough will land you on top of the flagpole.