How do I "set origin" without it messing up my mirror modifier?

Currently I am applying a mirror modifier and currently I am in object mode

As you can see the origin point is way off so I want to set the origin based on geometry.

Currently this is how I have the mirror modifier setup:

image

So this is how I set the origin point:

When I do this, this is what happens to my model:

Why does this happen and how would I fix this?

It happens because the geometry is only half of your complete object, and so is stuck in the middle of that half. When you mirror it, the mirror is therefore overlapping.

The best solution would not be to mirror based on the origin at all. Instead, whilst the origin is where it is, use Shift S and select cursor to selected. This should put your cursor at the same place as the origin. Then add an empty, which should be added where the cursor is, and then in the mirror modifier, use that empty as the mirror object. You can then play with the origin to your heart’s content without breaking the mirror.

You may want to make the empty a child of your object, so that it moves with it.

Did I set this up correctly?
image

If so then how do I now get the origin in the middle of the object?

You have to remember that the whole that you see is not the geometry. Half of it is created by the mirror modifier. If you want the geometry in the centre of the object, go to edit mode, select the loop around the centre (mirror) axis, use shift s - cursor to selected. Back to object mode and then origin to 3d cursor. (Note- if you do it this was you can dispense with the empty and go back to mirroring the old way, since the origin will be at your mirror axis.)

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