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First off, if you have a P-Wing in your set of items, definitely use it here, it'll make the level MUCH easier. If you do, just fly towards the top of the cave so you don't get hit by anything and fly up in the right little opening in the ceiling. (Read below for further instructions)

Now, if you don't have a P-Wing, then your going to have to get to be the bigger-sized Mario and then you'll have to get the feather, so then you can at least have the ability to fly. Towards the end of the cave (far right side of it), clear your path so you have a good open space to run. You should see a Koopa in there. Hop on him once so he gets in his shell. Pick it up, and then run to max out your P-Bar, and go up the right opening in the ceiling. The opening will go off into the right and will have a couple of bricks and two of those plant-eating things (whatever they're called). Toss the Koopa Shell in at the plant-eating things and wait for the shell to kill them and hit the bricks so that it can leave. Then, land in there and go through the pipe and continue the level.

Yeah, it's pretty frustrating. I remember it took me several tries. Just hang in there and eventually you should see what to do