How do I draw separators?

Use a static control/window with the SS_ETCHEDHORZ style (This is what explorer does and should give you the most accurate rendering when it comes to Visual Styles)

I don't suppose you're just looking for LineTo?

For menus and toolbars, generally the separators are drawn for you by the menu and toolbar APIs. For random separators in dialog boxes, etc, like in the picture you added you can just draw a line.

OK, you just want to draw straight lines on the form. That is trivial. When drawing your window, i.e. when processing the WM_PAINT message, simply draw a line using the MoveToEx and LineTo functions.