A/C compressor removal/install

First time changing compressor and don't have a compressor gauge to extract any air in tank. I am going to buy the refill cans an the oil can. Saw a video stating both cans go into the same line. So I was wondering if the refill line is were I drain the tank or is there another location to do so, and if I can stick something in the valve stem an let the air spray out.