Circuit Training is the last puzzle in the list (though probably the third to last puzzle you'll actually do in the game). It will be located in the Airfield in the Hangar (after completing Happy Skies).
*** Spoiler - click to reveal ***To complete Circuit Training, drag the red circuit on top of the green one, interlocking their edges. (1)
Drag the blue circuit down, dragging it up next to the red circuit but putting it all the way down. (2)
Move the purple circuit over to the left-most side on top of the blue circuit, against the wall. (3)
Drag the red circuit up to its connecting port. (4)
Drag the green circuit up to its connecting port. (5)
Put the purple circuit between the green and red circuits. (6)
Connect the blue circuit to its proper port. (7)
Finish the puzzle by connecting the purple circuit up. (8)