It's not that hard: just get access to some sort of bench mounted
belt sander, hopefully with a selection of belts. For breaking
edges, I'd probably just use a 220, or perhaps a worn out 220 grit.
You don't need to chew off a whole lot of metal, just enough to
soften the edges a bit. If you can find one of the generic 'skinny'
benchtop belt sanders, (the ones that use 1x42 belts) those work
well because they have an area high on the belt that isn't supported
by a steel backplate. That 'slack' area will let you get softer
profiles than you would have by grinding against a steel backplate
that forces the belt to cut only flat facets.
To actually dress them, for round faced hammers, I normally cradle
the hammerhead in my left hand, and rotate or spin it about its long
axis, while touching it to the belt at about a 45 degree angle to
start. Doing this high up on the belt where it's slack helps also
because otherwise the handle gets in the way. There is no one
"official" position or technique. The final goal is to take that
hard, sharp 90 or 45 degree edge on the edge of the face and soften
it out to a nice rounded shape. Any way you can get there is fine,
and "how rounded" is entirely up to you, and what you want the
hammer to do. Since you're asking this question, I'm guessing you
don't have anything specific in mind for these hammers, so I'd stay
pretty close to a rounded version of whatever edge they've got on
them now. You can always grind more off later.
One handy trick for polishing them is to do the final grind with a
worn-out 220 belt, then polish them with a buffing compound called
"stainless". (I got my last brick from Allcraft.) It's a weird chalky
white compound that absolutely devours steel, and with the right
buffer will take the edges of a hammer from 220 to a mirror in less
than a minute. Run it on a large (6"+) treated stitched buff, and
you should be fine. Stainless doesn't do much for precious metals,
but it's a real lifesaver on steel. I polish my planishing hammers
with it, and I can read in the reflections. Remember to rotate your
directions as you buff, or it'll dig dragmarks into your steel just
as quickly as it polishes.