You'll need 35 gems to unlock this level. If you've cleared all three other episodes completely, you should have 192.
As in Super Mario 3D World, there are several Conkdors here. The first one you see is surrounded by coins. Ahead are platforms moving in and out of the wall. There's a ladder here which allows you to reach the top of the wall. There's a clear pipe that will drop you down to the wall behind the first Conkdor, which has many coins. Move forward and you'll find a clear pipe that will take you to the next area. Use the pipe to get around the Conkdor surrounded by quicksand, then run past the four Conkdors lining the path. Ahead is the oasis, with some coins in the pool. Cross the shifting path, then cross the narrow path with the Conkdors. On the other side is a Conkdor surrounded by bricks. Lure the Conkdor into breaking the brick in front of the pipe. Go through the pipe for a Gold Mushroom. Drop down and go to the other side of the ramp for a Conkdor and the Green Star.
Special Challenge: Collect the Gold Mushroom.