I have never had a problem with ordering from Connecting Threads, altho I have never made one of their kits.
There are a few things that you can check on your own, to help you succeed on your next attempt. A very basic one, but really also the most important, is: Are you stitching a consistent 1/4 inch seam? Are you using a 1/4 inch foot on your machine or are you relying on the seam guide on the bed of your machine? Sew a few pieces together and check the width of the seam. You may need to move your needle a little bit to get it exact.
Secondly, you did not say if you cut the squares yourself or used a pre-cut bundle. Not all pre-cuts are created equal. I have had some that vary as much as 3/8's of an inch in the same bundle. If you cut them yourself, just a few threads more or less, here or there, can make a big difference in the end.
The third thing I would look at, is the difference between 'ironing a seam' and pressing a seam'. Pressing means putting the iron on the seam and lifting it off. Ironing involves moving the iron along the seam or fabric. Ironing can stretch and distort the fabric. Using steam can shrink some fabrics and stretch others. Ironing may have been the major culprit in your first attempt.
It is great that you are going to try again. A simple kit with explicit instructions would be the next best thing to a quilting class. Good luck and this board is always here to help. They are an awesome bunch!