The idea here is basically the same as the nursing style tank, except we have sleeves to deal with, which makes things a little more difficult, but not too much. If you need more detailed instructions, check out the nursing tank tutorial. I will try to abbreviate this one a bit to save myself some writing. Here is the finished shirt:
You will need:
2 t-shirts of the same size
Scissors
Thread
Sewing machine
3/8" elastic
Put on t-shirt #1 and cut two vertical slits, one over each side of your bra. Put on t-shirt #2 and mark with a pin 2 inches below your bust line.
Zig zag around the openings of t-shirt #1, making sure that the zig zag is off the fabric for half the stitch. Pull on the t-shirt as you sew so the zig zag will have some stretch to it.
Now start cutting. You will cut the side seams (all the way through the bottom of the sleeves) on both shirts.
Then you will cut the sleeves off, following the seams.
Lastly, cut the shoulder seams on both shirts. You should have 2 fronts, 2 backs, and four sleeves. Discard 2 of the sleeves and one back (or save them to make something matching for baby :).
Now, take t-shirt #2 front and cut it off at the length you marked with the pin (2 inches below bust). Pin a hem by folding the fabric over 1/4", followed by 1/2", all the way across the bottom of the shirt (the folded part should be on the inside of the shirt).
Stitch the hem close to the upper fold (about 1/16"). Then cut a piece of elastic the same length as the hem if it won't need to stretch, or the actual size of your under-bust front measurement if the shirt will need to stretch.
Using a safety pin attached to the end of the elastic, guide it through the hem.
Now, stack the shirt fronts, right side up, with the cropped front on top of the longer front.
Now stack the shirt back (wrong side/inside up) on top of the fronts. Pin shoulders and stitch together, using a 3/8 to 1/2" seam allowance.
This next step is sort of tricky. Open up the shirt and lay it on the table (right side up). Get your sleeves and pin them onto the shirt. Make sure to hold them up to the arm opening before you start pinning. On my shirt, the front part of the sleeve was smaller than the back. After they are pinned in place, stitch the seam.
The last step, which I didn't take a picture of, is to stitch the side seams. Turn the shirt inside-out and pin the side seams all the way through the sleeves, then stitch using a 3/8 to 1/2" seam allowance. Be sure you get all of the layers in the stitch. It can be sort of tricky when working with so many layers, so watch carefully. And there you have it :)