Nobody is obliged to fight a fire. The first decision is whether to try at all, and it is a decision about your escape route rather than about the fire. If using the extinguisher would put the fire between you and the way out, the answer is to leave, raise the alarm and let the fire service do it.
How do you use a fire extinguisher? Before you start
Has the alarm been raised and is everybody leaving. Is the fire still small enough that one extinguisher could plausibly deal with it. Is the extinguisher rated for this fire, checked from the label rather than assumed from the colour. And is your escape route behind you and clear. If any of those four is wrong, the extinguisher is not the right tool.
The PASS fire extinguisher method
Pull the pin, which breaks the tamper seal. Aim the nozzle or horn at the base of the fire, not at the flames, because the fuel is what needs treating. Squeeze the handle steadily. Sweep across the base of the fire until it is out. On a burning liquid, lay foam gently across the surface or onto a nearby vertical surface so it flows over rather than jetting into it.
Who should use a fire extinguisher?
Anybody may, and nobody must. Fire safety law requires employers to nominate sufficient people to implement firefighting measures and to train them, so most workplaces have staff trained to use extinguishers. Training is what turns an extinguisher into equipment rather than decoration: the most common failures are aiming at flames rather than fuel, and using it from too far away.
When to stop
Stop when the extinguisher is empty, when the fire is not visibly reducing, when smoke is building between you and the exit, or when you feel the heat increasing. Get out, close the door behind you, and tell the fire service what you tried and with what. A half used extinguisher is not a failure; a person who kept going past the point of leaving is.
Powder fire extinguisher use, and why indoors is the wrong place
Powder is the most capable portable extinguisher on paper: it knocks down burning solids, flammable liquids and flammable gases, and it can be used on electrical equipment. That is why it is the default on a forecourt, a van, a boiler house or anywhere outdoors with a mixed risk. Indoors it is a poor choice for reasons that have nothing to do with putting the fire out. Discharging it in a room fills the air with a cloud that removes visibility almost at once, which is a serious problem when the point of the exercise is that people are leaving; the powder is an irritant to breathe; and it gets everywhere and is abrasive, so a small fire in an office becomes a specialist clean and a pile of ruined equipment. In an occupied building the standard advice is to avoid powder in confined or restricted spaces and use water, foam or carbon dioxide according to the risk, keeping powder for the outdoor and vehicle risks it is good at.
Other questions people ask
What does PASS stand for?
Pull the pin, Aim at the base of the fire, Squeeze the handle, Sweep across the base. Aiming at the base rather than at the flames is the part people most often get wrong.
Am I required to use a fire extinguisher at work?
No. Employers must nominate and train sufficient people to implement firefighting measures, but no individual is obliged to fight a fire, and the trained answer is often to leave.
How close should I stand?
Close enough that the agent reaches the base of the fire, which for most portable extinguishers is a few metres, and never so close that the fire is between you and your exit. The label gives the discharge range.