Now, I realize there are other ways to do this but this is how I do it and I didn't think of this entirely on my own. I learned part of this from the owners of this LQS during a Mystery Weekend a number of years ago.
First, I press the fabric with sizing (not starch) that I purchase at a dollar store for $1. (I saw it the other day at Walm*rt for less than $1.) I then spray one of the fabrics with a mist of the sizing and layer the fabrics right sides together and press. The sizing will help to "seal" them together while I cut them and then until I sew them.
I carefully move the fabrics still layered together to my cutting mat and cut my strips with the fabrics still in layers. I have the light ends turned up just so you can see that I cut the layers together. (I am cutting 1.5" strips and sew with a 1/4" seam allowance.)
I keep the fabrics together when taking them to the sewing machine and sew the strips together like I would for any 9-patches. (Two sets of strips: dark/light/dark and one set of strips: light/dark/light)
After sewing the strips, I press the fabric toward the dark fabrics. Now I take the strips to the cutting board and line them up to cut. I will be cutting a complete 9-patch block all at once.
It doesn't matter what order you put the strips in. Put your first strip down on the mat a little to the left of a vertical line and exactly on one of the horizontal lines. In my picture below I have a pen pointing to my fabric lined up on the horizontal line. You will see that my selvages that I will be trimming off are to the left of the vertical line.
Then take your next set of strips and lay it down on top of these strips but below them in the same manner. It doesn't really matter how many inches you go down just so you put them on a horizontal line.
Then, again I do the same thing with my third strip. I now have all three strips lined up on horizontal lines and just to the left of a vertical line.
Now, if you look on the ruler at the spaces between 4-5, 7-8, and 10-11 you will notice they all line up at 1 inch intervals. Also, if you noticed, when I was laying the fabric down that at those 1 inch intervals is where the finished seams are at. (Every time that you use this method you might not end up with your ruler exactly on the "inch" spots like I did. However, whatever your measurement should be of the finished seam inside of your 9-patch it should be consistent and you can measure that on your ruler. I hope that makes sense.)
Once I am positive everything is absolutely straight I trim off the excess to the left of the vertical line. I am looking at the lines on my ruler, my mat and fabric for this cut.
Since I am cutting 1.5" strips I move my ruler over 1.5" keeping my eyes on the lines on the seams of my fabric strips (in this case the 4-5", 7-8", and 10-11" spaces on my ruler) and the left side of my cut fabric. I am not really paying attention to the lines on the mat except to make sure the fabric is staying straight on the horizontal line at the bottom strip. Once I know everything is absolutely lined up then I cut.
I continue on across the strips in this manner cutting complete sets of 9-patch blocks. Sometimes I may have to stop and even up my cut edge because things can get out of skew - even 1/16th of an inch. If that gets ignored it will only start to magnify as I go across. But usually I only have to do this once in the middle of a long strip. Let me say that I don't hurry with this step and am careful to not bump or move the fabric in any way. I make sure with each cut that every thing is line up perfectly before I cut.
I know I only need eight blocks for my swap but I went ahead and made whatever my strips allowed. The extras will go in my 3 1/2" parts box for future projects. Below is a picture of my finished 9-patches. I press them with sizing after pressing them open so they lay nice and crisp. They are all exactly 3.5" unfinished. Yeah!!
I would love to know if you find this technique helpful to you. I still have four more sets to make. :)
Happy 9-patches to you -