1 relatively older minifig torso; 1 Viking helmet horn; 1 battle droid arm, Exo-Force arm, or any other kind of arm you want on.Step 1:
You have to have an kind of old torso like the blue one on the left. This won't work with the red torso on the right.
Step 2: Remove the arm on the side that you want the robotic arm on.
Stick the Viking horn into the arm hole as shown in the picture. Make the tip of the Viking horn go into the opposite arm peg.
It should look like this:
This picture is really messed up, but if oyu look closely you can see how the tip of the Viking horn goes into the peg of the left arm.
Step 5: Attach the desired appendage to the peg of the Viking horn.