Put on gloves, a safety mask and clothes that you don't mind dirtying. Breathing charcoal powder can be detrimental to your lungs, and crushing the charcoal will make a mess.
Break up the pieces of charcoal with a hammer. Use a rolling pin to further crush the broken pieces, grinding it to powder. Don't worry about fully pulverizing all of the charcoal --- a mixture of small lumps and powder is ideal.
Spread the crushed charcoal over the soil you wish to fertilize to a depth of ¼ inch. Rake the ground to mix the charcoal in, working carefully if you've already planted your vegetables or flowers.