Remove your old broken fire brick and lay it on a flat surface
We sell the universal fire brick pictured below. https://stovedoorglass.com/product/universal-cut-your-own-fire-bricks/
Place your old brick on top of your new universal fire brick and mark around the outline of the old brick
How to cut your own fire bricks – Using a standard wood saw, carefully cut along the marked area. (This will not damage your wood saw)
Replace your new fire brick!