how to make an 'S' Shaped projectile in 2D ?

you can use bezier curve to move your projectile along the path.

How I would go about this is split the forward and sideways movement into two parts. One part that just moves it forward and a second that moves it sideways.

Here is an example script I wrote:

 /* Public Variables */
 public float s_amp = 0.01f;
 public float S_speed = 1f;
 
 public Transform target;
 public bool tracking = false;
 
 public missleSpeed = 1;
 
 /* Private Variables */
 Vector3 targetLocation;
 float timeAlive;
 
 void Start () {
     targetLocation = target.position;
     transform.LookAt (targetLocation);
     timeAlive = 0;
 }
 
 void FixedUpdate () {
     // Forward motion
     MoveToPlayer ();
     
     //Do the Snake motion
     DoSMotion ();
 }
 
 void MoveToPlayer () {
     // Rotate towards target of tracking
     if (tracking) {
         transform.LookAt (target.position);
     }
     
     // Move forward
     transform.Translate (transform.up * Time.DeltaTime * missleSpeed);
 }
 
 void DoSMotion () {
     // Calculate the S motion
     float sideMotion = s_amp * Mathf.Sin(timeAlive);
     
     // Increase the time alive
     timeAlive += Time.DeltaTime * S_speed;
     
     transform.Translate(new Vector3(sideMotion, 0, 0));
 }