I don't have all the nifty screenshots Kiya does, but I will try to tell you. <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />
You need points in the Enchant Weapon skill. Click on an item with a charm quality and a window will open and you will see the slots the charms go in. You just drag the charms to the slots and they will be locked into place. You can not remove them once inserted, though. The number of open slots will depend on the number of points you have in the Enchant Weapon skill and the charm quality of the item.
I hope that helps. <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />