Select Deadly Gift as your right click function, and a menu bar will pop up with icons for all of the trap types you have in your inventory. Left click on the type you wish to use. Right click somewhere on screen to release a trap. You will see damage and experience points scroll on screen when traps do damage or kill an enemy. Unlike resurrected creatures, they will not turn hostile if you get too far away.
You do not need to separate traps individually. However, you can do so by dragging a stack (of anything) into (or out of) your inventory (or a container) and holding the control key down while dropping. This will bring up the split stack pane, which will allow you to specify how many to transfer.
In games started after installing the 1.34 patch, Finnigan in the cellar of the Ducal Inn in Verdistis sells traps and other items.
For the elven graveyard, they leave in a cut scene, so it is very unlikely traps will be triggered.
