Java doubly linked list pointers to previous nodes

I'm making a method to add a Node into a list called "public void add(int index, T value)".

This method will put a value into an index, and then will have pointers to the next and previous elements in the list. I have messed up on the pointers to the previous nodes, which I have been sitting and experimenting but don't get to make it work.

Example: We have a list with Integer values [2, 4, 6]
Instance variables:
Node head, tail;

int amount, changes;

Instance variables for the inner class are:
T value;
Node prev, next;

Input:

add(1, 3);
System.out.println(list.toString());

System.out.println(list.backwardsString());

Expected output:

[2, 3, 4, 6]
[6, 4, 3, 2]

My code so far:

public void add(int index, T value) {
if (index < 0 || index > amount) {
        throw new IndexOutOfBoundsException("Index is not between 0 and amount!");
    }

    if (value== null) {
        throw new NullPointerException("Value can't be null!");
    }


    if (amount == 0) {
        head = tail= new Node<>(value, null, null);
    }
    else if (index == 0) {
        head = head.prev= new Node<>(value, null, head);
    }
    else if (index == amount) {
        tail = tail.next= new Node<>(value, tail, null);
    } 
    else {

        Node p = head;  
        for (int i = 1; i < index; i++) {
            p = p.next;
        }
        p.next= new Node<>(value, p, p.next);


        p = tail;
        for (int i = amount; i > index; i--) {
            p.prev= new Node<>(value, p.prev, p);

            p = p.prev;
        }*/
    }

    amount++;
    changes++;
}

In this occasion I would also ask how does

p.prev.next or p.next.prev

work?