Try pressing as you go. Establish a straight line on your ironing board: use a long metal ruler, cover your ironing surface with a fabric with a woven stripe or take a thin cord and tie it to both ends of your ironing board. Seam 2 strips together. Align the edge with whatever you've used to get the straight line and carefully set the seam. Open the seamed piece up and press, again using your guide to keep it straight. I usually press with the seam to the dark, but you can also experiment with pressing it open. Take it back to your machine and add the next strip, press and repeat.
I saw this on a you tube video, but I can't find it now...