Satin is a beautiful exotic fabric to work with. It has good draping capabilities, which means it doesn't have a lot of innate stiffness to it. Because of this, additional stiffness must be added in parts of the clothing. This is done by adding various materials called interfacing. You can use commercially manufactured interfacing with satin or you can sew another compatible fabric to it. Both will give the finished satin clothing structure where needed.
Lay the pattern piece of the part of the garment requiring added stiffness on the fusible interfacing and cut it out so that when you lay the fusible side of the interfacing against the backside of the corresponding satin piece, they match up. The interfacing will be heat-sealed against the backside of the satin.
Unpin the pattern piece from the interfacing. Cut 6 mm (1/4 inch) of the interfacing away all the way around. This is done because the standard seam width when sewing a garment is 1.5 cm (5/8 inch). The interfacing is cut smaller because the finished seams shouldn't have stiffener in them, as the fabric won't lie properly on the body when the garment is being worn.
Set up an ironing board and plug in the iron set on low heat. Lay the satin piece that is to receive the interfacing shiny side down on the ironing board. Lay the interfacing on the satin so they match up and the glue side of the interfacing is facing the satin.
Centre the interfacing on the satin. Pick up the iron and use the tip of the iron to make an X in the centre of the fusible interfacing to tack it to the satin. Test to make sure the interfacing has fused to the satin when the X was made by lifting the interfacing up on one end to see if it is stuck to the satin.
Carefully work the iron outward from the centre, making sure the interface remains centred. Heat up the interface until it is completely fused to the fabric.
Take lightweight organza that matches the colour of the satin and pin on the pattern piece for the part of the garment requiring interfacing, for instance the neckline, to the organza and cut it out.
Unpin the pattern piece from the organza. Pin the cutout organza to the back of the matching satin pattern piece.
On the sewing machine sew a 1.2 cm (1/2 inch) seam all the way around. Make sure the organza lies flat and doesn't pucker.
Cut the thread to release the fabric from the sewing machine. Take the scissors and clip away the 1.2 cm (1/2 inch) seam allowance of the organza as close to the sewing machine stitches as possible. Remove excess organza all the way around, being careful not to cut the satin fabric it is stitched to.
Fusible interfacings come in different weights and some are stretchable. If you aren't sure what kind of interfacing you want or need for your satin garment, buy a minimal amount of a couple different kinds and go home and test them on scraps of the satin fabric. If you are sewing using a purchased pattern, read the back of the pattern envelope. It will suggest the type of interfacing to use. Interfacing is usually added around necklines and sleeve openings and inside collars and lapels.