Be sure to set your seam first, that is put your hot iron on the seam before you open it, I just press, not back and forth but up and down to the end of the seam. Then lift one side of fabrics run your iron tip along the seam, this irons your seam to one side; I usually press to the darker side if at all possible.
Sometimes I use spray starch and sometimes not, just according to the fabric and how it behaves.
Good luck,
delma