After you sew a seam, you press, not iron, the seam while it is closed, just like it was sewed. That sets the stitches. Then it depends on if you are joining the piece you are making to another piece. If you are, you probably want to press, not iron the Seam going in the opposite direction of the adjoining one so they nest. This is where you want to watch a couple of YouTube videos on 'Nesting your seams for quilting'. Otherwise, press, from the front, if you prefer, toward the darker fabric so the seam won't show through on the top side. There are no rules, just helpful hints. You don't have to do what I say, or anyone says. That's what's so cool about quilting.