I've been hunting through mine and my husband's wardrobes for items to refashion and have come up with a few things.
This t-shirt of my husband's is way too small for him now and it looked simple to alter so I decided to start with it.
I removed the sleeves, then laid a t-shirt of mine on top and cut it out down the sides and along the bottom, allowing extra for sewing up. I hemmed what was the sleeve seam to form cap sleeves and sewed up the sides.
It wasn't until I'd started cutting it out that I noticed the side seams were twisted. There was no way I could straighten them up, so I cut it out as it was, which resulted in a slanting bit of the old seam at the bottom of each side. After sewing the new side seams, this formed a small wedge shape at the sides which I emphasised by sewing a button to the top.
I finished the neckline with a small tab and a couple of buttons, and hemmed the bottom.