On the sixth stage, the player meets for first time the InertBox and the rough terrain. The stage is not harder
than the fifth, but the player gets to know about the new box type and the new terrain.
The IceBoxes were introduced on the fourth stage and are not unknown to the player anymore.
Step by step solution
From the start position - go UP once, then push the InertBox to LEFT once.
Now push the IceBox UP once.
Go RIGHT, then UP.
Push the IceBox to the LEFT once.
Now you have ot use the InertBox as a stopper for the IceBox. Go LEFT, then DOWN, then LEFT, then DOWN again.
Push the InertBox to the RIGHT once, then go UP, then RIGHT, RIGHT, DOWN, DOWN and finally LEFT.
Push the InertBox UP, then go LEFT, UP, UP, UP, UP and RIGHT.
Please note that, if there is no stopper for the IceBox, it will simply pass the socket, but the player will not be teleported. Now push the IceBox DOWN once, to its place.
Congratulations. You have completed the sixth World. Get ready for the seventh.