Well, I’d have to say that “deciding to be a…….” is usually not a common approach. At this point it has likely been decided already, knowingly or not.
I’ve never considered myself a “sissy”, but I have explored the realm. I’m straight, love women and all thing’s wonderful about a woman’s body. My exploration was from a sense of wanting to be/experience what it felt like to have a woman’s anatomy. I have worn prosthetic breasts and the lingerie, baby doll nighties, panties, bras, etc.
While I never discovered myself as such, when in the particular mindset, it has been fun to play around. Do I do it often, not at all. Would I do it again, likely if I feel motivated as such.
Good luck in your journey.