How to power up arduino and two 6v motors with 12v LiPo efficiently?
If you want to use a '12V' (11.1V?) Lipo to supply 6V to the Arduino and motors then you need something to drop the voltage down. Perhaps not a regulator, but at least a device that keeps the output voltage relatively constant at different load currents. If you want efficiency then you must use inductive and/or capacitive components which have minimal loss, not resistors or semiconductors operated in linear mode. That leaves you with 3 choices:-


buck-mode switching regulator: This is not hard to make if you use an IC like the LM2576. One diode, one inductor, 2 resistors to set the voltage, input and output filter caps, done! If this is too hard then just buy a ready-made module.
step-down transformer: Requires a specially wound transformer, transistors on the input to create AC from DC, and a rectifier and filter capacitor on the output to convert the AC back to DC.        
Switched Capacitor: Connect a capacitor to the battery and charge it up, then discharge it into a larger capacitor. Repeat until it charges up to 6V. Requires several switching transistors and a complicated driver circuit. IC's that do this are available (eg. LM2771) but most can only handle low voltage and current.