Place a gear by selecting it from the inventory, make the gears turn at their target RPM, remove a gear with right mouse button. Use + and - keys to switch between layers. On the same layer you can connect gears by placing them next to each other, the gear will turn BLUE when at meshing distance. Gears on different layers can be connected by axle, place the gear on top of another gear to connect them with an axle, it will also turn BLUE when ready to connect.