Here's an example of strips starting to curve (see picture):
You can see the red and orange strips have a bow in the center, while the other strips are stright.
Here's the trick:
Watch the direction you're sewing. You don't want to sew from top to bottom, and then start again from the top and sew to the bottom to add the next strip. That's the reason you sometimes end up with bowed strips. Instead, you want to change directions with every strip that you add. So, for strips #1 and #2, start at one end and sew down. When you add the third strip, start with the opposite end and sew in that direction. By reversing direction every time you add a strip, you can make sure that your seams stay accurate and consistent.
So, how do you keep track of which direction you're sewing?
Our trick is to place a pin at the end we started sewing at. When we’re finished sewing those strips, we remove the pin, and place them at the other end, so we know to start there with our next strip. That way, if you need to take a break from sewing for any reason, you’ll know where to start when you pick your project back up!
So, next time you’re sewing long strips, follow our trick to make your seams straight and accurate! See a video of this process here.