When sewing a garment from scratch, always iron each seam individually as soon as it is finished and before sewing it to other portions of the garment. You should never wait until the garment is complete to iron the seams.
Ironing each seam as it is completed not only will provide a much easier enjoyable sewing process, it will also result in a more professional final garment. This is especially important when sewing seams with crossing seamlines or intersecting seams- the connection between these seams should remain completely smooth and flat as the parent seams are aligned and stitched. Ironing the seams one by one will ensure that every seam connection and intersection is smooth and accurate which facilitates the dressmaking process.