Start out with two big T-shirts, the bigger the better!
Cut straight across the shirt from one armpit to the other.
Cut the bottom part into two parts (or more if you want more stripes). I tried to make them about the same height as the top part of the shirt for symmetry. Do this to both T-shirts.Take one of the pieces and pin it onto the top piece, right sides together. Sew all around.
I did these steps to both shirts. I started out wanting the green on top and then changed my mind and wanted gray on top. You can make it easy on yourself and just sew together the bottom two pieces of one of the shirts.
Measure how wide the opening of the sleaves are (at the hem), take that measurement x 2. This is how wide a rectangle you cut out from the “sleeve shirt”. Measure the height by trying on the shirt and measuring from the hem of the short sleeve to where you want the new hem to be.
Sew the rectangle onto the short sleeve with the opening being where the seam on the sleeve is.
Sew following the lines and cut off the excess fabric.
Cut out your new neckline as low or high as you like (or keep the original).