I took a t-shirt that fits me nicely to use as a guide. I lay it over top of the t-shirt.
and cut around it leaving an inch seam allowance on all sides.
I then pinned around where I cut and sewed up the sides. Because I left the existing neck line and hem lines all I had to do was sew the sides.
After pinning it I realized I would like it better as a short sleeve so I cut off the sleeves. I didn't hem them to finish them. I just left them as it.
It took about an hour to do and I think it looks super cute. I love the fit of it, it's one of my favorite t-shirts now.