It's pretty easy to do. The Form class derives from Control, so you just need to add the form instance to the Controls collection of the Panel you want. Oh, but only after you set the Forms TopLevel property to False. Just remember that your still using a Form. You still have to Show the form to get it to show up!
MyForm mf = new MyForm();
mf.TopLevel = false;
Panel1.Controls.Add(mf);
mf.Show();