I'm pretty sure this is how the flat seam is made... I don't know the technical name.
sew the pieces together with a straight stitch and then pull out the double eye needle (I got one with my machine, you may need to go out and purchase one).
It looks like this. iron the seam to one side and pin. then go over the seam with a running stitch with the double needle making sure the outside of the fabric is facing up. You'll need two spools of thread, one for each eye.
i hope that helps a bit