I originally read about this in Bust magazine. I could not find the instructions on the site, but I probably just didn't look enough. Besides that I gave away some of my older issues, so I am just doing this from memory.
First I took a tshirt that was way too big. It's one that my mum-in-law got for my husband, but it's an XL and too big for him. I was wearing it as a sleep shirt, but then I had seen the article in Bust and put it on the side for this project.
I took a tank top that I like and placed it on top of the tee. Then, I took a sewing marker/pencil and traced with dashes around the tank. For the front neck line, which is lower than the back, I placed pins around it.
Then, I pulled back the tank and marked where the pin was through the shirt, removing them as I went.
Next, I took my handy dandy hem ruler and went completely around adding a little more than a half inch to the outline that I traced.
Cut that baby out around the outside set of dashes.
Turn it inside out. When I cut, I actually cut to where the tops of the shoulders were still attached by the original seam. It makes it a little bulky, but oh well. I then pinned over around the neck line and arm holes and the bottom seam. I sewed those areas first.
finally, I pinned the sides together and sewed those up.
Here's the finished tank.
The issue had how to make a pair of knickers from the same tee. I have not done that yet. I may see what style I want to make and sew those up too. I probably would take my time to make the inside areas prettier/neater next time, but for sleep ware, hey it's cute!