Explain how to overload .ToString() method...

To overload, you can create another method with the same name but with a different signature:

//same name, additional boolean parameter
public string ToString(bool upperCase)
{
    return upperCase ? this.ToString().ToUpper() : this.ToString();
}

If you actually mean override the ToString() method you can replace that method by doing the following:

//override of Object.ToString()
public override string ToString()
{
    //put your custom code here:
    return "ToString() from override";
}