There honestly isn't a way to 100% guarantee you'll die, there's always something that could go wrong. Although there are some ways that will highly increase the chances of success.
I think a shotgun in the mouth aiming towards the brain stem is basically a guarantee. Also doing full suspension hanging with a support and rope that can support more than twice your weight as well as making sure you won't get found for at least 30 minutes is basically a guarantee you'll die.
Sodium azide (SA) doesn't have an antidote, so if you take several times the LD50 and an anti-emetic regime so you don't throw up, that should also work.
But, even then, things can go wrong.