A sinker as it is thrown in the higher levels is usually a two-seam fastball (index and middle fingers along and over the narrow seams, thumb underneath on the horseshoe seam). To get good sink, you have to throw it hard and low in the zone with a good downward angle (if you get under the ball at release, it won't sink). Some pitchers move one or both fingers slightly inward from the seams to get more sinking action. One version, which some people call the "Little League sinker," actually has the fingers together in the middle of the smooth cover between the narrow seams. This pitch usually drops sharply but is slow and difficult to control. If you have good arm action, the basic two-seam grip should give you all the sink you need. Just a few of inches of sink on a two-seamer will yield many ground balls. Experiment with variations on the basic two-seam fastball grip and see what works for you.