Some fabrics are difficult to sew by themselves. Knits can curl at the edges, making it difficult to run the fabric through your sewing machine. Other fabrics need added structure to make the finished clothes drape properly. Machine or hand embroidery goes much more smoothly when the fabric is reinforced, keeping it from slipping around while you work. Attaching interfacing fabric to the back of your main fabric will reinforce it and add stability.
Cut the interfacing fabric according to your pattern so that you have a piece of interfacing for each pattern piece.
Pin the interfacing to the wrong side of your main fabric. If you are using fusible interfacing, iron the two fabrics together and remove the pins when the two fabrics are fused together.
Sew the pieces of combined fabric/interfacing according to your project instructions.
Not all interfacing is the same. Select an interfacing fabric that has similar qualities to your main fabric. Test the interfacing's compatibility with your fabric by laying a swatch of interfacing with a swatch of fabric to judge weight and color.