Yeah, the mouth is really something I need to work on.
Released a new update yesterday btw.
Append in the armor (shift+f1), Use G to move the armor and place it carefully on the character rig. (Holding shift while moving will move the armor slower, and press x, y, z or mousewheel to move it in a specific axis. Just some tips to help move it more precisely)
Then to parent the armor:
1. Go into pose mode with the bones for the character rig.
2. Select the armour
3. Hold shift while selecting the bone you want to parent the armor to.
4. Press P
5. Select "Object"
And you should be good to go!
Hope this helped, if you're still having problems, let me know!