I've been doing it for months. Everything I've done to prepare has brought up issues I needed to deal with, and has helped me to determine if ctb is indeed the right choice for me or not. Things that helped me were researching and planning methods, writing goodbye notes and then realizing it was for the best in my situation to not do so, and reading Stoic philosophy about the rational reasons to suicide. If you use the site search with the term "Stoic" or "Seneca," you'll come up with a lot of stuff. If you put in my username with the search, you'll see my threads and comments in particular, especially the Stoics' five rational reasons for withdrawing from the party of life.
In the past, I did call emergency services, because I wasn't really ready and I thought they could help me. Now, after all of my preparation, I am mentally strong enough. My only concern is the suffering of one method that I am certain will work but have fears of one symptom, so I am working on ways to mitigate or eliminate that symptom. If I cannot find a way to do so, then I'll do a lot of meditating, such as visualizing and pretending I am experiencing the symptom, reminding myself that it will pass, and that within a set amount of time I will lose consciousness. In all other regards, I am calm and grounded, so I will now work to bring that to this symptom, but it's tough. I don't want to be rescued, I don't want all the shit that will come along with it, so I won't attempt that method until I'm certain I can make it all the way through.
To me personally, there is a difference between what I want vs. what I know is the best choice for me. Ctb is the best and most rational choice for me. I've accepted it. I'm no longer worked up or upset about it, and that's because of months of processing, much of it here on the forum. I'm not running toward ctb, but want to do it when the time and conditions are right. I've already attempted twice in my current state of calm acceptance and feeling prepared, with methods that failed, but it was nothing traumatic or dramatic, and there was nothing to recover from. I am already certain, but if the method, timing, and conditions are not right, then for now, I have the tools to manage daily life as it is until conditions get worse, so I don't feel pushed to act before I am ready. But all of this is just me, my situation is not like everyone else's, so I'd say to take from this what serves you, and don't worry about what doesn't. You are unique as well, and you'll figure your own stuff out. Keep asking questions.
Also, in my time on the forum, I've experienced and observed that facing the possibility of suicide and grappling with it -- researching methods, working out logistics, writing letters, etc. -- makes suicide smaller and allows things to emerge that it might have been blocking when it was bigger. Doing all this takes away much of its power. Suicide is a tool, a means, not an entity. Some folks do all this work and the things that arise help them discover ctb is indeed the right choice, while others discover something they really needed to work on or would rather pursue than ending their lives.