Filling the overflow to the correct level will suffice as the radiator will draw out what it needs.
The cap is on the plastic overflow bottle on the firewall.
The bottle on the firewall with the radiator cap on it is where you add coolant.
The overflow bottle may overflow if: 1. The car is overheating - this means after the coolant in the radiator gets heated up and expands it flows to the reserve bottle but the radiator doesn't cool down to suck back the coolant from the overflow bottle, but just keeps on sending coolant to the bottle. 2. You may have put excess coolant in the bottle - hence heated coolant from the radiator didn't get enough space.
Bad radiator fluid.
no antifreese
Coolant is added to the overflow bottle instead of the radiator. The overflow bottle will allow fluid to fill over into the radiator.
It is on the overflow bottle.
Depends what type of car, but generally it's near the radiator.
Engine is overheating.
assuming you don't have a cap on the radiator, you pour it into the overflow bottle. on my 03 it's located on the passenger side of the engine compartment, against the firewall.
On the coolant overflow bottle
The radiator cap may be at fault.