We are attaching the front and the back pieces above where your arms would be. Pin and sew from the top of the neck down to the edge of the shoulder on the right and the left side. I used the widest zig zag stitch on my machine (7mm wide) and a 1.5 or 2mm stitch length.
Sometimes I'm super lazy and don't pin my fabric before sewing it, but that's a terrible idea with stretchy fabric like this. You may find that sometimes you have to be careful to make sure that the top piece of fabric is feeding into the sewing machine as fast as your bottom piece. I was able to do this by stretching my fabric a little when I was feeding it into the machine. If you have a lot of trouble, the internet recommends a walking foot attachment for your machine.