One of the most recognizable pitch in baseball is the 4 seam fastball and a sinker pitch is quite similar to it. When you throw a 4 seam fastball in baseball you need to grip the baseball across the seams with your index and middle finger. Your ring finger should be touching one of the seam while your thumb is across one the bottom seams. Sinkers on the other hand needs to be gripped just like the 4 seam fastball but just with the seams instead of across them. Also, you will need to pitch it on the arm side of the pitcher and down for that downward trajectory.