The first thing is to check the plastic container that contains the reserve refrigerant liquid. Except in special models (German and Swedish cars), you can add coolant even when the engine is somewhat hot. When opening it (always covering the hand with a wet cloth), check for any resistance or let steam escape. If this happens, the tank is under pressure and you should not tamper with it until the car has cooled completely.
Next, check the radiator. Open it slowly, always with the wet cloth in your hand. Be very careful, because there is a risk of burns due to steam or very hot coolant. If there is a lot of resistance when trying to open it, let the engine cool down for a few more minutes.
When examining the radiator, carefully look inside to see how much fluid you need. If possible, respect the water / coolant ratio suggested by the manufacturer. After filling the radiator, cover it and verify that the hoses that communicate with the rest of the engine are not broken or leaking. If everything is in order, start the engine again and pay attention to the temperature needle; If it threatens to reach the red zone, turn off the engine immediately.
If conditions permit, drive very carefully until you reach a service station or a mechanical workshop. If adding water and refrigerant does not seem to solve the problem, it is likely that the source of the problem is due to other factors, which may include: a blocked radiator, broken or not working fans, leaky hoses (sometimes not at sight), or broken water pump. But remember that the main thing is to stay calm and follow the aforementioned tips.