I'm going to put a regular brick wall in front of the old stone wall.
They will be seperated by about 4 inches and the gap will be filled with
concrete.

The old stone wall is 3.5 feet high and has no real foundation. The base
of the bottom stones are about 2inches below ground level and rest on
crushed stones. The wall is stable and been up about 20 years.

Behind it is soil that goes to the top of the wall. On my side I see the
full length of the wall.

I'm putting in front of the old wall flags that will go about 18 inches
below ground level and stick proud of the soil level by 6 inches. So,
along the length of the old wall at the front will be flags rising 6
inch from ground level.

The new front brick wall will be laid adjacent to these flags. The
inside edge of the new wall and the front edge of the old wall will be
about 4 inches and it will be filled with concrete.

Okay, that's the general setup. How deep should I make the foundation
trench for the front brick wall? And how deep should the cement be in
that trench? Do I fill the trench with cement first to ground level and
let it set and then build the wall on that? Thanks.