Before you do anything else, press the fabric open. Then, align a square ruler to your cutting ruler to ensure the cutting ruler is at a 90 degree angle to the fabric.
I am now ready to cut my 3" strips. Flipping the edge of the fabric to be cut to my left side, I place the ruler onto fabric making sure the 3" line is on the fabric, not outside the edge of the fabric. If the 3" ruler line is not onto the fabric, every time I cut the fabric the strips will be a little bit narrower than the measurement I desire. By the time I sew them together, the strip set may end up being 1/4" to 1/2" smaller than I need them to be.
Sally Collins's book is full of extremely helpful tips for machine piecing blocks which, when assembled, look perfect together. As with anything else, the more you practice the better you get at it. However, the process becomes more enjoyable when you have great resources, such as this book, at your fingertips.
Try it next time you sew strips together, and you will be pleased with the results. Have a fun weekend!