While flexing itself can slightly increase muscle size over time (roughly 4% according to some studies), physical growth mainly comes from consistent training.
Lift your chest upward toward the ceiling. For a more "elegant" look typical in women’s physique, try a slight twist to make your shoulder (delt) and tricep pop at the same time. Training Tips for Better Definition teen girl flex
To get that "solid flex," focus on these standard techniques used by fitness creators: While flexing itself can slightly increase muscle size