I often imagine doing it with a shotgun, more of an intrusive thought though like when I'm lying down to go to sleep. That or just stabbing my neck. Neither of those are my chosen method though.
My chosen method I imagine more intentionally, going to get my materials, setting up my N2, making the exit bag, positioning and securing everything, and following the procedure of turning on the gas, hyperventilating, pulling the bag down. Feels very mechanical and emotionless when I imagine it.