Decals are already rigged to work. You just need to open a texture and it should work.
If you want to apply a face to the rig, basically what you do is remove the eyes and mouth, select the head and switch over to edit mode, then select the face that will have the texture applied, and make a new material and assign it to that face. Then just make a texture for that material and load one up.
If I remember correctly that's what I did. If you want to be able to edit the expressions but use another face, do everything I said except removing the eyes and mouth, but you also need to edit the face texture and remove the eyes and mouth from that in Photoshop or whatever you use.
