It starts outside of a couple of Yoshi houses. There are berries hanging from the roof, as well a Baby Red Yoshi. There's also a text box that simply reads Yoshi and his print.
The first house has some bushes with lots of berries (though impossible to collect as you wouldn't be able to bring Yoshi through the door) and an apple. Otherwise, you can just leave and head to the next level(s).
If you go into the second house, you will find a hole and a text box that reads: This is my trash heap. Hey, absolutely no falling in!!! ABSOLUTELY. This is like a bad joke... Jump down the hole and you will fall into a hidden sewer level. It is made up of lots of pipes and water currents. There also is lots of outlined switch blocks (even though you wouldn't need the solid blocks once you hit their respective switch, as well this level leads to one of them). Eventually you should come up to a water fall that leads up to the surface. Swim up it and you will now be in another room with the same message as before. Outside is the secret goal.