Charcoal can be activated with calcium chloride, zinc chloride, lemon juice or citric acid.
Take 1 liter of water, dissolve 250 g calcium chloride or zinc chloride, or two teaspoons of citric acid, or 1/2 cup of pulp free lemon juice.
Add powdered charcoal until it is a slurry with a little excess liquid. Or if you have a smaller fixed amount of charcoal you want to activate, add the solution to the charcoal until it is a slurry with some excess liquid.
Let it sit overnight in a covered container.
Drain off excess liquid using a large basket coffee filter in a large strainer.
Rinse well with distilled water and drain three times.
Then you can either air dry or dry it in an oven.