The trick is this:
With a ball of clay in your hand and a clay shaper
tool in the other, capture a part of the face and when
part of it is right on target, stop.
Cure what you have that's right.
Make a mold of that right bit you captured.
Cure the partial face mold.
Make an impression of raw clay and start working on
another part of the face.
That way if you wreck the part you got right, you can
make another impression of it and begin to work on
another part of the face again.
There is nothing more frustrating than getting part of
the face right and then when working on another part
of the face you wreck the good stuff you had.
Curing and making a mold of the good bits, and do that
in stages, makes sculpting a known face easier.
Use left over polymer clay for your "Iteration" molds,
because you're not going to keep them around, they are
part of the process to be discarded as you progress.