Open Level Editor (“Tools - Level Editor”)
Press the “+”
Click Edit Level
Set the first layer size between 6 and 10
Press the Add Layer button
Scroll the Scroll wheel of the mouse down
Draw the layer holding LMB and hovering above the layer grid, use RMB to erase
Repeat steps 4 to 6 until you have enough layers
Make sure that the number of items in the level is divisible by 3 (if it is not, you will be notified by a warning)