How did I do it - You ask?
With the pillow case laid flat on a solid surface; the opening towards the bottom, I laid a favorite tank top on top of the upper portion (closed seams) of the pillow case as a template for the neck, shoulders and armholes
the armholes are still unfinished which is why I'm wearing a sweater - that and it is February - Only Californians (Ok, and Hawaiians too) would dare wear a tank top in February.
I took in the side seams by a a total of seven to nine inches total to give it that slim-fitting look you see above
With the excess fabric from the side seams, I finished the neckline
I liked the length of the pillow case so I decided not to alter that. It was a comfy, casual look I was going for... helps hide the Super Bowl Yummies I ate AND depending on how its worn, gives it an automatic shirring look which is always flattering.