I started but slicing them down the vertical center line with a ruler and rotary cutter.
For the simpler of the two projects, I simply placed WRONG sides of the raw edges together and sewed them with about an inch seam allowance. The large seam allowance made the finished shirt a smaller size. Because I was sewing two different colors together, I had white thread in the bobbin, and black thread on the top of the machine. I sewed the back and the front of the shirt the same way. Sewing wrong sides together will leave a raw edge exposed. If you don't like the exposed raw edge, you can sew these together right side to right side and the seam will be on the inside.
For a second, more complicated variation, I measured six inches in from the cut edge of the front of the t-shirt and marked a line 6 inches down the length of the shirt.
I then cut strips about 2 inches wide to the marked line.
After repeating with the black side, I stitched the backs of the shirts together. Then I wove the strips on the front together, twisting each strip before pinning it down. I ran a single line of stitching down the center of each strip. Here is a photo of the inside to show the stitching pattern:
You can see that when I tried it on, twisting the strips left some gaps, and the two topmost neckline strips didn't want to play nice.
So I carefully ran a line of stitching down the center of the woven section, making sure that the strips overlapped each other to close the gaps. I carefully cut along the neckline with scissors to reshape it.
Ta da! Two new funky t-shirts that fit!