How do you get your camera to focus in on subject with lens extender on it?

use manual focus

Same way as you would without - turn the focusing ring. Remember, though, you'll have to have the camera very close to the subject - you are doing close-up (macrophotography), after all.

Lens extenders can refer to two different things. One is like a "hard" bellows that puts distance from the lens and the camera causing the camera to focus in a "macro" mode. The other can be a 1.4x, 1.7x or 2.x optical focal length extender making a 200mm (at 2x) a 400mm minus two stops.

If the optical extenders are compatible with your lenses, there should be not need to do anything but auto-focus the the lens in the normal way you do it with out the extender between your lens and camera. If on the other hand you are using a "spacer" so you can close focus, then you will most likely have to manually focus the lens, sometimes moving the camera back and forth to obtain focus.

It depends on the AF module of your camera.
Most will not work with lenses of a smaller maximum aperture of f8.

Look a the ring around the front element of your lens, it will give an f-number, if it is f4 of a higher number , then by the time you add a 2x convertor then your lens if f8 wide open.

Most cameras will focus at full aperture then stop down the lens at the moment of exposure, if your lens if f8 wide open then it cannot open any further.

You will need to manually focus.