Wood ashes are a good source of potassium, calcium, and several other trace elements. However, it is fairly alkaline, so shouldn't be used on lime-hating plants.
Just sprinkle it around your plants and let the rain (if any) wash it in.
(Although many people used to do it, ashes from coal fires are not recommended to be used in this way, as they can contain heavy metals and other potentially harmful chemicals.)
