In my opinion, it's not about thinking but doing. A lot of people, including myself, try or have tried to think themselves into it, but it's impossible. I think if someone's serious, the way to go is: 1) practicing and rehearsing the process, the setup, 2) actually starting to make final arrangements (goodbye letters, cleaning, etc.), 3) watching gore videos to desensitise and reduce fear. Things like that... Obviously, the person needs to acquire the accessories, find a location, etc., which is also, in essence, doing stuff.
These might still not help, and the person still might not be able to pull the trigger in the very last moments – but what I've realised is that if I just keep thinking about it, then I'm definitely not going to get there, ever.
What's interesting is that doing things in preparation also makes me realise the seriousness of my intent, and can trigger intense fear – which, in turn, makes it possible to deal with and push through those fears.