You have found the same problem ALL of us have had at one time or another. No big deal, it will go back to being fun just take a deep breath. When I'm going to cut strips I try to make sure that I've starched my fabric before cutting. This helps keep your fabric chrisp as well as keeping the bowing down. Now when you go to sew a strip onto your rectangle (rectangle being larger and face up with strip face down - with starch or without just work with what you have), sew together (walking foot helps a lot or pinning a lot helps too). Now press (PRESS not iron - please), your strip open, with your two pieces sewn together these will be placed face up and your next strip will be face down. This is when you will turn your block around (so you will be sewing your seam in the opposite direction), new strip is always on top facing down and you change the direction of your sewing (ie: if your rectangle is pointed with the outer edge away from your machine when you sew your first strip down, then it will be turned with the outer edge facing your machine for the second strip to be sewn on - again facing down). Most of the time when you are finished you will need to square up a little (some blocks more some much less). Good luck and breath