Here's an article with a few tips on how to do it the right way.
Make sure the engine is cold! Hot antifreeze burns dramatically and it will burn you, too!
Anti-freeze contains a lot of chemicals that do more than protect your cooling system from freezing. Most good brand name anti freeze products contains anti-corrosive chemicals which are especially important to engines that contain aluminum parts. Therefore it is a good idea to flush out the coolant each season or at least every other season and replace the anti freeze to gain the protection from the new fluid.
Most engines have drain plugs on the block so that you can ensure that you drain all of the old coolant. These plugs are usually very hard to get to as they are hidden behind the exhaust system and other components on the engine. As a result most people don't drain the block. If you want to make sure you get all the old stuff out you can flush the system with cold water, then run the engine until the thermostat opens (feel the top hose and run until it gets hot) and then shut it off and drain it again. If you do this a few times you will basically remove all the old coolant by a dilution process.
If you can get to the drain plug, (sometimes it's hard to get to) you can put a piece of 3/8 inch hose onto it. Put the other end into the drain pan and open the drain plug. This will let the fluid empty into the drain pan - that way all the fluid will go into the drain pan and not spread out and drip all over the place. Remove the radiator cap.
Now comes the rub. You have pure water remaining in the heater core and the block so how much anti freeze should you add to ensure that you get the correct mixture (50-50)? Look in your owner's manual and find out the capacity of your cooling system. For example let's assume your system holds 8 quarts. The simple answer to the question is just add 4 quarts of anti freeze and then top it off with water. But what happens if you can't fit 4 quarts in the system due to the amount left in the block and heater core?
Here's the trick. On the last draining, drain the coolant into a drain pail and measure the amount of liquid you were able to remove. Let's assume you could only get out five quarts - that means there are three quarts of pure water left in the cooling system. In order to ensure that you have a 50-50 mixture of coolant you need to add an equal amount of anti freeze so just add 3 quarts of anti freeze to the system. Now you have 6 quarts of 50-50 mixture in the system. In order to make up the difference just add 2 quarts of 50-50 mix (one quart of water and one quart of anti freeze) to the cooling system and it will be topped off with the correct mixture of anti freeze and water.
When you arrive at the auto parts store, you are likely to see plenty of brands of antifreeze. Most antifreeze falls in two categories: Regular and Long-Life. Your vehicle owner's manual will specify a particular type of coolant and an employee should be able to assist you in finding it. Regular antifreeze is green in color, while long-life antifreeze is orange or pink. Long-life antifreeze must be used in vehicles that are designed for it and may be substituted when flushing, for regular antifreeze. Along with the various types of antifreeze you will notice an assortment of cooling system treatments.
A word of caution! Antifreeze/summer coolant is very toxic. Don't get it on the body paint, or on your skin. Wash it off with water! Just what does one do with the old coolant that is drained from the system? You can call the local conservation office and get a recommendation from them. Check the local laws in your community. Call 800-424-9346 to find out the latest recommendations from the EPA as to how to dispose of antifreeze. One way is to dilute the antifreeze solution with water at a ratio of 10 to 1 and dump the coolant down the sanitary sewer drain. It seems that the bacterial in the sewer effluent will break down the anti freeze into components that can be handled by the local disposal plant and will not harm the environment. DON'T DUMP IN DOWN THE STORM SEWERS. IT IS TOXIC IN ITS ORIGINAL FORM AND WILL KILL ANY ANIMAL THAT DRINKS IT! It also stands to reason that you shouldn't leave it standing around in open containers since a dog or cat might drink it - if it does it will die within a day unless it is treated by a veterinarian!
