One of the most elemental rule sin dressmaking is that each seam should be ironed right after stitching and not after the garment has already been constructed. it is this essential step that will make the difference between a professional garment and one that is home made.
For a conventional way to iron seams, follow these simple steps:
1. As soon as you machine stitch the seam, iron the stitch in the direction it was applied. For example, if you applied your stitch starting at the neckline and moving toward the garment's hem, the stitch should be iron in the same direction- from the neckline to the hem. Doing so eliminates possible tension in the finished seam, while also smoothing and flattening the machine stitch.
2. Unless otherwise specified, iron the seam allowance layers open using the highest heat setting allowable for the fabric at hand. It is easiest to iron the seam allowance open from the wrong side of the seam as this give you more control in the ironing process.
3. To further smooth out the finished seam, iron again on the seam's face side. It is important to be vary careful when ironing on the right side of the garment especially if working with heat-sensitive fabrics. If working with such fabrics, always use a protective cotton cloth as a barrier between the hot iron plate and the fabric's surface underneath.