I have a set of flagstone covered steps going to my backyard. They are all loose. When I looked to remortar them I noticed that there was no base underneath. To do this correctly I figured that I would have to put down a concrete step base, then mortar to that; or use bricks or blocks.

If I decide to go the concrete way, the problem that I am seeing is that setting up a stair stringer for either side is way beyond my skill level. The reasoning is that either side of the steps is a stone wall that has curve to it. Are there tricks for doing this?

Another consideration that I have is does the concrete get poured right up to the stone? Leaving a gap might cause a problem with freezing water.