Stand 6 to 8 feet away from the fire and follow the four step PASS procedure recommended by the National Fire Protection Association:

P – Pull the pin and hold the extinguisher with the nozzle pointing away from you.

A – Aim low, at the base of the fire.

S – Squeeze the lever slowly and evenly to discharge the extinguishing agent. When the agent firsts hits, the fire may briefly flare up. 

S – Sweep the nozzle from side to side, moving carefully toward the fire. Keep the extinguisher aimed at the fire.