I exhaled through my nose when I smoked. It kind of bothered me that I did...I used to try to catch myself and not do it because I didn't think it was very good for my sinuses. I didn't succeed. When I started vaping, at first I ALWAYS exhaled through my mouth. It was like I felt like I couldn't exhale out my nose for some reason. Now I do, though, and with vaping I don't worry about my sinuses. I just do a "mash up"...a little of both mouth and nose exhale. I don't know exactly how I "learned" to control it. Probably the mirror thing would be the best bet for starters.
I'm a girl, too.