The pitcher grabs a baseball and finds the area on it where the seams are the closest together. Then, the baseball is rotated so that those seams are perpendicular to the pitcher's body, with the index and middle fingers on each of those seams.
Each finger should be touching the seam from the pads or tips, almost to the ball of each finger. The thumb should rest underneath the ball in between the index and middle fingers, finding the apex of the horseshoe part of the seam. The thumb needs to rest on that seam from the side to the middle of its pad.
The two-seam fastball is designed to have more movement than a four-seam fastball, but can be more difficult to master and control. This ball will tend to move for the pitcher a little bit depending on velocity, arm slot angle and pressure points of the fingers.