How do you make a cut off shirt with huge arm holes?

so ya I'm wondering how to make a cut off shirt with those massive arm holes. and not the ones that make it look like a tank top I'm talking about holes that go from ur shoulder to a little above the bottom of the shirt. I see girl athletes wearing them alot and I need some so when I go to our open fields for hi school soccer I won't boil in my clothes this summer. the girls usually wear them with sports bras underneath and the shirts are cute and obviously cool enough to wear in direct sun. sooo... how do u make them? is it just as simple as flattening the shirt and cutting the sides to the size u want? or..? advance thank yous :):)