Get a salad bowl of the hottest tap water you have. Stand the can upright in it as you fill the system. It will not be too hot, due to the can cooling as you fill. Put in a bit, and close the valve to check the system pressure again. It usually takes a couple of cans if you're pretty low. It's pretty fool proof if you are careful.

AND, you don't turn the can. You fill gas into the low side with the can upright. The hot water helps evaporate the liquid R134a, and helps empty the can.