How to remove black circle from Raynox 250?

I was out testing out my Raynox 250 on my Nikkor 18-300mm this morning and I got this black circle effect when shooting. Could anyone show me how to remove it please?

What is causing the circle effect is that either you're seeing the inside of the lens barrel, or the image circle (the light coming out of the rear of the lens) is too small to cover the entire area of the sensor. I can't remember exactly which, but I'm guess that it's the later. If it's the image circle, then no light is hitting the outer edges of the sensor, which is why they are now showing as black. This only happens when using too wide of a lens (too short of a focal length).

SOLUTION: Zoom your lens to a longer focal length and the black area will disappear.

It's called vignetting. Nothing you can do about it. It's physics - the curvature of the glass elements and the length of the barrel of the lens.

I assume that a Raynox 250 is a lens hood, and thus is indeed vignetting at the wide-angle end of your 18-300 lens.

If it is a "converter" to lengthen the reach of your lens, then note that Raynox was always a cheap brand, and results from cheap brands are always uncertain.