The top is the CINO Nessie top pattern using the colorblock option. That's right, I didn't even have to hack the pattern. It comes this way - ready to embellish!
Now, I didn't think this up myself to be honest. I saw this pin that doesn't really lead to a tutorial or anything, but I did want to give credit. The pinned example looks to be made of felt, but I thought it would work well with knits. I've kind of got a thing for reverse applique so this really charmed me.
This technique is pretty simple but gives big results. You will need to cut 2 pieces of that upper bodice in contrasting fabrics. Most any knit will work. These happen to be a cotton/spandex and a rayon/spandex underneath. As long as it doesn't fray, you are in business. Stack them on top of each other right sides up. You can baste around the edges if you like, but on a small piece like this, I didn't bother. (You could also cut 2 rectangles big enough for your pattern piece, do the technique, and then cut out your actual pieces from your new "fabric")
Use a ruler with a 45 degree angle to mark lines across. I used chalk and spaced mine 3/4" apart.
Sew across those lines with a straight stitch. I chose matching thread but imagine the possibilities. I used a stitch length of 2.5 as usual, but larger would be cool too.
Now mark the lines the same distance in the opposite direction and then sew them too.
Now comes the scary part. You will need to use some small scissors to cut away the 2 bottom sides of each square. Gently pull the 2 layers apart and pinch the bottom corner. Picture a vertical line running down to that point at the bottom. Make a small snip at the center bottom. If you centered it well, it will make it easy to then cut up to the corner on each side. The first few will feel scary as heck, but you will get quicker and more confident after just a few squares. I've done other reverse applique projects (you can see them here) where I completely cut away that top layer so it didn't matter how neat or perfectly placed that first snip was. With this, you are leaving the whole top layer there so you do need to be as accurate as possible with that first snip. You could always practice on a scrap first if you are really worried.
Then just sew up the pattern as usual. I waited to cut open some of the squares that were near the edges until after I sewed the rest of the shirt. I didn't open a couple at all if I thought they might stick out funny near a seam.