How to access method from another class?

I have main form and another class

how can I access from another class?

example:


public partial class Form1 : Form
 {

     public  void log(string msg)
        {
            listBox1.Items.Add("[" + DateTime.Now.ToString("yyyy/MM/dd HH:mm:ss") + "]" + " : " + msg) ;
        }

}

another class :

some codes here:
        {
            Form1 _form = new Form1();

                _form.log( "example");


            }
        }