Just like in a car, you don't use 100% antifreeze, so first, you'd need to know the volume of water in the system. Then, drain out enough water and replace with antifreeze. You need one designed for heatting systems, and you may want to replace or add (if it doesn't have one) a backflow preventer on the supply line...antifreeze is not something you want to have a chance of getting into the potable water. So, it's somewhat hard to say without seeing it.
The second question, for me anyway, is why do you think you need antifreeze? While there are some systems that may require it, most do not. Using antifreeze in a heating system decreases the heat transfer, making the boiler functionally smaller. SO, keep that in mind, too.