Add where you normally would, except, wait until it cools down some. Use caution when you remove the cap and turn your face the other way so that the hot steam does not get all over your face. If possible, use a gloved hand or rags because it will be extremely hot. Prepare to pour your correct mixture of antifreeze and water (seek owners manual) and run the engine while adding. If you don't run the engine, you could crack the engine block.
Your engine is too hot.
Engine coolant.
The water and coolant is put in the coolant reservoir for the location of this refer to the owners manual. It is best to do this when the engine is cold otherwise you run the risk of getting boiling water all over you. the mixture of coolant to water should be about 50/50.
To raise the boiling point of the coolant.
in the coolant resivor
When the engine is cool
Pressurizing the cooling system raises the boiling point of the engine coolant. under normal conditions this keeps the coolant from boiling and having to be frequently topped off.
Coolant recovery tank
In the coolant reservoir in front of the engine.
1) Your engine is overheating and the coolant is boiling. Or 2) Exhaust is leaking into the coolant, probably through a bad head gasket or cracked head.
The TTR125 uses an air cooled engine, there are no radiators to put coolant in.
The bigger your car, the more coolant you need to put in. The bigger your engine and car, the more coolant you need