A CBT modality for CTBing? Becoming an extreme pessimist and filling your life with continuous constant reinforcing negativity. Self-isolation, making daily reminder flashcards of all of the reasons life isn't worth living with post-it notes all around reminding oneself of everything that you most desire but will never have. Essentially you'd have to create your own psychological echo chamber. Becoming a philosophical pro-mortalist, as the rationale for why death is a safer state to be in then to exist, where each day one is exposed to a broad array of harms which are likely to arise in the future if they haven't an already.
After the conditioning stage and being firm in certain on death, you'd have to stop engaging the brain and act for rehearse. As if there's a death warrant or you summoned yourself to Capital punishment and command yourself to commit to the decision. That's the hardest part, and that's the gap between idealization and mortality. Reducing a thought to practice against primal instinct
Because SI is really a composition of fears and reservations but not a single instinct since there are nine definitions for instinct in science, one would have to overcome the fear of the unknown, the fear of the process the fear of death, contend or work towards a truly effective method with it almost impossible survival.
In most cases, SI is overridden chemically by anything that would suppress the fight or flight response and promote capacity for reckless actions by inhibiting the parts of the brain responsible for self regulation. Think of alcohol; remember the last time way back when you did something when you were really fucked up and drunk that almost got yourself killed or seriously injured? I'm sure we all have a memory of doing something under the influence that was so stupid and careless in hindsight that we're lucky to say we survived it unscathed. So there are chemical and substance induced methods of inhibiting the brain from acting on certain thoughts, overthinking kills the deal. Once you engage the brain you're fucked. So they would have to be an element of chemical inhibition and impulse.
Those two would also require a CTB ready go kit with the lethal means or protocol on hand or nearby somewhere accessible in a kit that one could grab and go when you catch a moment of impulse or even just a really horrible day.
According to publications and suicidality, these are some of the most significant elements which in combination, result in the highest rate of successful attempts somewhere in the order of 95% +. Because most suicides I believe 54% are Parasuicide attempts. The person does not actually want to take their lives they want to be taken seriously as a form of what's called signaling or mitigating some extreme pain but with the reservation of Hope.
I'm not sure if I'm not sure what your objective is or what brings you here but if life is not for you or you truly renounce life no matter how marvelous it can get and with rational consistency in your repudiation of it with a fixed intent on CTBing, then it would be like apoptosis or programmed cell death of the psyche. An organism slowly begins to shut down by cascade signaling domino effect, and in the psychological, sociobiological and psychological voluntary case it would be a process of progressive of conditioning death in a suicidal inducing mixed processes anchored in a strict pro-mortalist pessimistic philosophical/Nihilistic self-induction of accelerating death by self-deterioration and decay.