3 things work for me
1. Visualize it, feel it, get used to the idea
2. Take your preferred drug or substance
3. Use a method you can ease into. OD, hanging, something you can ease into. Even on the day you're going through with it something you can start, get a feel for, calm yourself as you contemplate if this is really what you need, something that doesn't require force that can just naturally happen with minimal effort from you
Im not sure if I will ever be able to just grab a gun and shoot myself but I'll definitely sip on some sn