Mortar required for door jamb

Hi all, I am looking for some info/advice on what to do to create an even surface for installing a door jamb on an external door. I had to remove the old door jamb, as it was rusted on the inside from a moisture leak in the roof. While removing the old jamb, the mortar used underneath broke away quite easily, creating an uneaven surface. Should I cut the bricks back with a grinder to create an even surface, or should I use mortar in between the brick gaps to create an even surface for installing the jamb? If you suggest mortar, what type should I be using for this scenario?