so I accidently drain the transmission fluid on my 2012 camaro... how can I add the fluid back???

You will have to find your transmission fill plug, above the pan midways on the drivers side of the transmission. I would suggest using new fluid, transmission fluid is very susceptible to picking up particulates.

http://www.camaro5.com/forums/showthread.php? t=120571&highlight=l99+automatic+transmission This is a good step by step DIY.

The torque converter holds about a gallon too, so the unit has to be running to circulate this out, else there is a drain plug where you can drain the converter without hydraulic assistance.

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so I mistakenly drained transmission pan while attempting an oil change about 3 qtrs. came out of pan the vehicle was not running obviously. My question is do I add 3 qtrs. back into pan or do I need to drain entire system and start over.