Charging tools or crystals can be quite similar to cleansing, just with adding in energy rather than removing unwanted or excess. You will find some repeated methods from the Cleansing Guide, just tweaked a bit for the charging aspect. Some people find using chants or rituals to work the best, but the most important thing in terms of charging an item is having the intent that an element or the like will charge it. These are only some of the many ways, and this list may be expanded.

Air
Let the item sit out in the open air or wind. Non-polluted or Clean air is best.

Earth
Bury the item in fresh dirt, rocks or sand (i.e. no potting soil or boughten mix).

Fire
Pass the item through a flame. Smoke may also work, but actual fire is the most effective if you want the full element.

Water
Put the item in water. Natural sources would be the best, such as rivers, lakes, oceans, waterfalls, or even the rain. Please consult the Cleansing Guide on stones that do not work well with water.

Storm
Set the item out during a storm. Warning: Be careful not to lose your item if you choose to do it this way. If need be, keep it inside and let it rather be in the presence of the storm instead of in it. This is less effective in some ways, but much safer for some circumstances for you and the item.

Nature
Place the item in some natural surrounding (e.g. tree roots, grass, flowers, bushes, etc.). If you feel it necessary, bury it near roots.

Sunlight/Moonlight
Let the item be bathed in light for at least a full day/night. For the Moon, more rays or light reflecting off of it may prove more useful for you.