I have a couple of questions re tiling over my painted brick fireplace

(1) We demolished the raised hearth and want to install tiles. We find brick below where the hearth was with a 3" trench running in between the brick across the width of the hearth. My concern with placing tile directly over this is that the tile may be unstable due to the trench. Do I put concrete in the trench to fill it or instead use something like wonderboard across the entire width of the hearth ?

(2) I have seen a number of posts about tiling on a painted brick fireplace but unfortunately have not seen a detailed step by step for a DIY-er. I am ready to take on this job but would love to get some inputs from others who have done this before. If there is such a guide somewhere, pls help me find it.

Thanks in advance