Here's how I did it (and you can too).
I picked up an XL men's black t-shirt and grabbed my sewing scissors.
I cut the sleeves off and cut off the side seams.
Your arms go through the hole with the shoulder seam at your neck and the other seam at your back.
Here's how it looks from the back
(I can't believe I'm posting a picture of my backside.
Good thing it's a little blurry and cropped!)
And 5-minutes later she had her very own vest!