white fiberglass insulation

Two fold question.

I am installing/replacing glass doors on my fireplace. In the instructions it says just to install fireproof insulation between the frame and brick face of the fireplace.
It looked VERY ugly on the old one and I do not want to repeat the same problem.
How do I make sure there is no insulation showing up through the mortar gaps between bricks?
Should I fill in this gaps with mortar or some kind of fireproof sealant? How do i make it neat?
Again my goal is not only preventing a backdrafts from the fireplace but also making it less ugly.
Also, if someone knows how to clean up brick from the soot?

Thanks,

Mik_mik

 

Just in case my post was missed...