For the direction I wanted to go with this, I knew this old blue, ribbed, Anthro tank with lace trim would be the perfect partner in crime.
I tried it on first, drawing in chalk approximately where I wanted the pieces to fit together, and made sure to cut below that line.
Then piece two got the axe. I cut off the arms/chest/collar all in one piece, thinking I may use that portion for another refashion someday.
It took the power of my whole brain to figure this puzzle out: right sides together, top portion upside down, so when sewn and flipped up, full article will be correct. (Yikes.)
Starting with the underarms in place, then front center and back center, I stretched the tank to match the button down, like you would with elastic*, creating a bit of gather.
A zig-zag stitch seemed right to use to sew these two together, so there would be a little give, and then I trimmed off the excess.
*In hindsight, I would not do this exactly the same. The knit fabric pulled too much in the back especially, and had a funny, puffy effect. I suggest not to stretch the tank portion, but pin the dress shirt to it in the same manner. But, rather than taking the stitches out and starting again, I created a few little darts to get the back to lay more flat, which helped, and was quicker solution.
All in all, I think this is a completely wearable summer top. Luckily for me, I have more dress shirts to play with, so I'll have ample opportunity to put to use the lesson learned here.
...and in the meantime, I'll stop and smell the roses.