Kathy, Stamping curved surfaces is definitely a problem. The stamp
is a flat surface. When held against a curve surface only parts of
the stamp will touch metal. The stamp should be used against a
surface that is flat along the stamp axis.
The stamp must be made in an area where the backing of the project
touches the block you stamp against. If you stamp against a surface
that is recessed on the front of the piece you will put a dent in
your work. I find the best surface to hold the piece against when
stamping is a polished steel bar. Most of the time you will mar the
front surface that is between the stamp and the stamp block. It is
best to stamp before you do the final polishing.
You should practice on scrap silver until you can hold the stamp
still and hit it with a solid blow with the hammer. The hammer
should be heavy enough to deliver a blow sufficient to drive the
stamp into the metal. I find that a slight tap followed by a
forceful blow of the hammer works best. The tap sets the stamp and
the blow drives the stamp into the silver.