Figure out how much the quantity of recommended coolant capacity is , for example 4 gallons to fill the engine radiator etc. 50/50 mix would 2 gallons 100% coolant and 2 gallons of distilled water. Start by adding 1 gallon of coolant ,1 gallon of water,1 gallon of coolant then the remainder of the 1 gallon of water. Some times all will go in, some times you can have left over water. then go from there.