in your inventory, hover your cursor over it. at the bottom left of the screen it will tell you what to press to re-charge it. upon doing that the right side of the screen will bring up a list of full soul gems. select one from the list and hit re-charge. the little bar on the screen will show you how much each gem will charge the item.
It's too bad they didn't carry over the Oblivion option to pay certain folks to recharge items, would be a decent money sink when using powerful items.