The mirror modifier in Blender is a great tool for symmetrical models like the one we are creating because everything can be created in only one side and copied to the other. Now is a good time to add this modifier and take a look at how our model will be when it's complete. Before the application of an extrusion modifier, we have to make sure that the center point of the object is really at the center.
Change the view to see the model from the top, and select all of the vertices at the right of the model, as shown next (section A). This selection can be made with the border select tool, which is the B key.
Press the S