After you defeat Exor at Bowser's Keep, you are transported to the Factory, in Smithy's realm.
Explore the Factory
From the factory, you can go back to the world map if you use the spring southwest of where you first arrived.
Go northeast and you will find a bolt and nut that goes over a gap. Jump onto the nut and keep jumping to make it move to the other side. If you fall off, you will be bounced back to the left side of the screen so you can try again. On the other side, save at the Save Block, then go northeast to the next screen.
Go northeast and you find another nut/bolt bridge. This one has a Glum Reaper floating nearby. Go northeast to the next island. Beware of the small monster hiding in the middle of this island. It's an Ameboid. It will get up and start following you when you get close. You can go across whichever bridge you prefer. The Glum Reapers over each bridge are going at different speeds. There will be a hidden Ameboid on each island after the bridge, no matter which one you cross. At the end, go northeast to the next area.
Ameboids will be bouncing around this area. Dodge or fight them, then jump onto the button in the north corner. This creates a bridge in the east corner. Go there, continuing to dodge or fight the Ameboids, and go northeast across the bridge.
Save at the Save Block, but before you go, stand to the west of the Save Block and jump around that area to find a hidden chest that contains a healing mushroom. Then go northeast to the next screen.
Hit the treasure chest to get the Ultra Hammer, a weapon for Mario. Then jump on the spring to get onto the bridge. Jump on the nut to make it go across, and carefully jump to the second half and go to the other side. At the end, jump on the spring. You will battle Count Down.