How to implemt in-place merge sort using linked list in C++?

I am new to algorithmic programming especially in C++.

I would just like to know the basic approach (with example) to implementing in-place merge sort using linked lists. There are a few conditions/requirements:

1) The data in the nodes cannot be changed, only the pointers can be altered.

2) The Mergesort(Node* &head, int elements) function takes 2 parameters, a reference to the head pointer of the node (&head) and the number of elements in the linked list (elements).

3) Recursive merge sort is preferred.

If someone can at least show me how to approach it with a psuedocode, I would greatly appreciate it.

Thank you.

   void Mergesort(Node* &head, int elements) {
   }

   void Merge(Node* &head1, int elements1, Node* head2, int elements2) {
   }