Start with a carbon-rich material such as coconut shells, wood, or peat
Dry the material completely
Heat it in a low-oxygen environment to make charcoal
Crush the charcoal into small pieces or powder
Mix the charcoal with an activating agent such as steam or carbon dioxide at high temperature
Alternatively, soak the charcoal in a chemical activator such as potassium hydroxide, phosphoric acid, or zinc chloride
Heat the treated charcoal again to open and enlarge its pores
Rinse the activated charcoal thoroughly with clean water until neutral
Dry the activated charcoal completely
Store it in an airtight container