Marry the seam is what my beginning teacher called pressing your stitch line before you pressed the piece open. It embeds the stitch thread into the fabric. I just press it flat without wiggling the iron and then turn back the small square from the top side and press it to the corner using the tip of the iron in the center of the stitch line. Some people use the side of the iron, but that part of the iron is curved so it doesn't press the seam as evenly. I personally use steam, but that's a whole new can of worms for some people.