How do you add break fluid?

Put it inside the brake master cylinder

Why do you think you need to add any? There are only two reasons the level will go down. One is when there's a leak. That means the problem must be fixed. Adding brake fluid is not a solution. The second reason for the level to go down is when the front disc brake pads are worn. As they wear, the pistons extend out of the brake calipers to take up the space. That is the self-adjusting feature of all disc brakes. Brake fluid fills in behind those pistons and the level drops in the reservoir. All mechanics know to never fill the master cylinder during other routine service such as oil changes because when the brake pads are replaced, the pistons have to be pushed back into the calipers and the fluid behind them will go back up to the reservoir and make the level go back up. If someone filled it previously, that fluid has no where to go except to run over onto painted surfaces and the floor. Brake fluid eats paint off the body.

Also be aware that we read way too often about people contaminating the brake fluid with a petroleum product such as engine oil or power steering fluid. That is a REAL serious situation and an expensive repair that would likely exceed the value of a 17-year-old car. Experienced mechanics even wash their hands before working with brake hydraulic systems to avoid getting fingerprint grease mixed in with brake fluid.