Patience.
There is some conditioning you could try, in short it is leading yourself. CTB by gun get to know the weapon, hold it. Other methods, just be familiar with the method. Time…SI will do what it is supposed to do, for some maybe there is a reason to draw out the decision.
This next is how it works for me.
Stacked up my pain in life till finally I buckled under it all. My SI kicked in but I just held my breath for a second and it became real clear I would not survive this one. Still have days that arent bad but I know where things are going and my time is limited. I got to an acceptance point. I will also most likely die by gun, so yep, a little scotch, good spot, a last moment to let my grief rip me apart and then all stop. I survive by dieing.