Start by choosing a sweatshirt that's a little too big. An old sweatshirt is fine, even with a design on it. I chose a sweatshirt a size bigger than I normally wear.
Lay the sweatshirt out nice and flat, front side up.
Lay a ruler across it and mark the center between the sleeves, at the armpits and near the bottom.
Turn the ruler so that it is along all three marks and draw a line down the center of the sweatshirt.
Now slide a cutting board between the front and back (so you don't accidentally cut the back) and cut the front open along the line.
Here are the three places I marked the center
Then I drew a line down the front
Here is the front cut open