Go to party camp and talk to the dwarf called Sandal. Tell him that you would like to do some enchantment. Then a window will pop up, listing your weapons you can insert runes into it. Press on one of the weapons to select it. Then the weapon will be presented at the middle of the screen with 1-3 sockets next to it, and all you have to do is drag your rune into that socket in order to apply it.