It’s pretty straight-forward, just call DoDragDrop in a MouseDown event. You’ll need actual files on disk for this to work.
private void Form1_MouseDown(object sender, MouseEventArgs e) {
string[] files = new string[] { @"c:\temp\test.txt" };
this.DoDragDrop(new DataObject(DataFormats.FileDrop, files), DragDropEffects.Copy);
}