The Grip: Grips can vary between bowlers and the intended delivery. The standard grip for a fast bowler is to have the index, middle finger and thumb running down the seam as shown below. It is worth noting, there are numerous variations caused by different grips which are discussed on the deliveries page.
For a legspinner, who is aiming to rotate the seam off the pitch, the standard grip is 'two-fingers up, two-fingers down'. The top joints of the index and middle finger are placed on the seam of the ball, with the ring finger tucked against the side. From here, the fingers should be able to give the ball a big flick by rotating the fingers anti-clockwise.
Offspinners rarely have the same grip as another but the general rule for the grip is to have your fingers running across the seam like a leg-spinner. The grip should get the fingers into an ideal position to turn the bowl clockwise like a doorknob. Usually, the offspinner gets most of their rip using their index finger.
While these grips are the standard and definitely a good starting point, the best grip is one which you feel comfortable with. No bowler has the exact same grip or action as another.