If using a siphon, you will need to develop a head on the line for suction. Since its coolant, I know I don't need to tell you to NOT use your mouth.
1) get a long enough piece of tubing that will go to the bottom of the tank and reach the bucket/tub you will drain into.
2) Fill the tubing up with fluid. Hold your finger tightly on the end of the tubing you will put into the bucket.
3) Insert other end of tubing into the tank.
4) The bucket must be lower than the tank. This will create a suction/draw.
5) Put other end of tubing (the one with your finger on it) into the bucket.
6) Release your finger and watch the fluid flow.
You made need to watch the top of the tube and move it around like a vacuum cleaner hose to the low point in the tank. Once you get air into the line, you will lose the suction and made need to resort to a turkey baster for the final bit, or remove the tank and dump whatever is left. Then install your new tank.
Don't get coolant on your paint!!! Clean immediately if you do!!!
When you refill, you may want to consider adding Water Wetter. It can help run a few degrees cooler.
hope that helps.