Check out the first link in my post above.
If you have an item with a charm quality equipped and click on it in the equipment window, another window should pop up with charm slots, that you can drag and drop charms into (move the charm window over so it is not in front of your inventory window, then click and drag a charm from your inventory onto the charm window and release it). The number of open slots in the charm window is determined by your skill level of Enchant Weapon and the charm quality of the item.

Once placed, a charm can not be changed or removed.