Put a knot in the thread and insert the needle inbetween the folded fabric. Put the needle through at point A. Pull the thread all the way through. Make sure the thread is secure in the fabric and the end of the thread is hidden inside the fold.
Directly opposite point A. insert the needle at point B. then slipping the needle between the fold of fabric, insert the needle back through the fabric at point C. Gently pull the thread to slide the two sides of fabric together.
The only visible thread should be between point A. & B. This will be a tiny amount of thread in a matching colour to the fabric, so it should be virtually invisible.
Continue until the section is completely sewn together.