I've been thinking about putting the gun in my mouth and angling it upward. How do you think that would work? I don't want to make a mistake.
I saw you read the thread on Guns, and I've replied to you a bit there.
GingerPlum is right in that if you err, you want to err on the side of aiming low.
• If you aim too high and miss the brainstem, you'll hit non-life-critical brain matter and turn into a vegetable.
• If you aim too low and miss the brainstem, you've at least got a chance of destroying the spinal cord directly beneath the brainstem, which is also effective.
In an ideal world, however, TAW122 is correct in that a slight upward angle is how you want to aim to hit the brainstem. That said, it can be very difficult to judge a 10-15° up angle when you're looking at the gun end-on, rather than from the side. You definitely want to spend some time first with the brainstem diagram in that other thread, and then in front of a mirror practicing, to make sure you're comfortable with your aim point.
I'm going to turn this digression into another warning:
DO NOT attempt to ctb with a gun without spending time practicing your aim point: aiming a gun is
not intuitive without considerable practice, and aiming in the wrong place can easily result in a non-fatal failure to hit the brainstem.