Don't think about the outcome.
As someone else mentioned practice shooting at the range. Especially with one hand (if you intend to do it that way, which you will probably have to). Practice minimizing recoil, or at least being experienced enough to expect it and compensate for it.
Don't do it at the range tho, that's a little cruel to the others. Also they will probably be more prepared to try and save you in worst case scenario (gunshot to head is not 100%, though shotgun is close from what i understand).
If you mean emotionally, how do you prepare for such a thing, you just have ot know that there's no option left. I believe in pro-choice, but i still believe in the RIGHT choice (and that such a thing is individual). It's easier for me to think about CTB because life is not worth living anyways.
Edit: as someone mentioned, it's not guaranteed. Shotgun shells will bring you closer, but you will leave a mess for those who care about you (if you care about that sort of thing). I have thought about a 40 cal round or anything bigger. In terms of aiming, apparently it is best for it to cross both hemispheres or hit the brain stem if you can. As others have suggested, you might want to look at another method.