Choose a plain t-shirt in your regular size (not larger) along with a meter of complementary patterned fabric (I always buy at least a meter of fabric since I’d rather have some fabric left over than not enough).
Use scissors to cut open the front of the t-shirt along your dotted line (be sure you don’t cut through the back!). Then cut away the ribbed neckband.
From your patterned fabric, cut a 4-1/2” wide strip of fabric that’s long enough to fit around the the entire front opening, running from the left bottom corner, up around the neckline and to the right bottom corner. Hem both sides of the strip by ¼”. Then fold the strip in half lengthwise, and pin it around the edges of the front opening and neckline so that there’s a 2” border on both the right and wrong sides. Once it’s all pinned into place, stitch it into place with your sewing machine.