Once you have cut the 45 degree angle off your strips, join them together so that the 45 degree angle stays true. You will need to sew from the angled end each time. This will give you a parallelogram shaped section as shown in diagram 2. (The straight edge may be on the left or the right side. Doesn’t matter which.) Don’t forget to press seams open.

 
Then you join the first and last strip together, making a tube. It will look a bit funny and out of shape at first. But once you lay it flat it ...

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