The sinker I recommend is a two-seam fastball, and it is thrown with exactly the same arm action, release point, and followthrough as the fastball.
Some pitchers get good sinking action with the normal two-seam grip.
Others move one finger in a bit toward the middle and off the seam, like a cutter. Depending on your mechanics, this can create a bit more movement, but it will also reduce velocity some. Start with the two-seam grip and experiment. The purpose of the sinker is to get ground balls. Throw it low, knees to mid-thigh, and hard.