Draw a rectangle, approximately 50% taller than it is wide, with the top angled slightly outward. This will represent the body of the shirt and the slope of the shoulders.
Draw a triangle flush with the left wholesale jerseys side of the rectangle, at the top. The triangle will be half as long as the football jersey, and the right side of the triangle will share a line with the left side of the rectangle. The top point of the triangle will be at the upper left corner of the rectangle. This is the left sleeve of the jersey.
Draw a small upsidedown triangle (base up, point down) inside the rectangle; this will be the hole for the neck. The base of the triangle will be flush with the top side of the rectangle, in the center, and will not run the full width of the rectangle. The length of the triangle (meaning how far it extends into the body of the rectangle) should be approximately half as long as the length of the sleeves.