One thing I have not seen mentioned and a common problem is make sure your fabric is folded straight. You will know it is not when your cut strips come out with a dog leg (V notch) in them on the fold lines. Rarely is fabric cut straight off the bolt, heck rarely is it even loaded on the bolt straight from the factory. So when you line up your selvages you may have to offset the cut edge as much as in inch (sometimes even more!) to get the fabric straight.
Here is a good youtube by Leah Day
https://www.youtube.com/watch?v=mkHLWTQSy1M