You can't overfill it. You just pump in new lube until it runs out the fill hole. It is important to have the car level. I use ramps on the front and then jack up the back until the car is sitting level. Make sure you use 75W140 or 80W140 FULL SYNTHETIC gear lube. I did not flush mine at all. I simply sucked out all that I could and refilled it. Due to how close the gears are in the bottom of the PTU, you will likely not get all of it out. As I recall, it only holds around 16 ounces and I usually would only get about 10-12 ounces out. It is nearly impossible to get the sucking hose to the bottom due to the gears inside.
It is best to do the job when the fluid is warm since it will come out more easily. However, do NOT do it when the car is hot. Otherwise you will burn the hell out of your hands on the exhaust system and other things. Don't ask me how I know that.