Exactly like I used to do with manual focus cameras and that goofy split-screen focusing gizmo.
IMO, that's the only way to manual focus and without it, manual focus is almost useless. So I never ever manual focus with my Nikon D300.
That's called a split-image rangefinder. A lot of former full-image Rangefinder users (read: mostly Leica users) wax poetic about them and mourn that no decent digital cameras have them today. They are kind of nice when you get used to them- especially full image rangefinders like the Leica's of old.