What is the conditions of modifier mirror? isn't true that modifier mirror should create first at the right side? anything else other conditions should to do?? hope will success always making while need modifier mirror.
It doesn't matter what side you choose. You need to know that the origin will be the center of the mirror. If you choose to model on the right side don't put vertices on the left. Enable the Clipping option if you want the vertices along the center of the mirror to stick to each other.