Let's think about writing a Python script step by step. 

1) Analyze and define the use case. What does the script need to accomplish? 

2) Analyze the required parameters? What does it need to know? How can it get the parameters? 

3) Define functions for use cases.

4) Execute the script if it is running as the main module.

Here is an example script for saving a message to a file:

import os

def get_user_input():
    message = input("Please enter your message: ")
    file_name = input("Please enter the file name: ")
    return message, file_name

def save_message_to_file(message, file_name):
    with open(file_name, 'w') as file:
        file.write(message)
    print(f"Message saved to {file_name}")

def main():
    message, file_name = get_user_input()
    save_message_to_file(message, file_name)

if __name__ == "__main__":
    main()

Write a python script for a user to append a message to a file. Provide your reponse without using the word file.
user