This is done with either a Dremel tool and a rubber abrasive shape, or some fine grit wet or dry sand cloth wrapped around a round or half round dowel rod or a file.
Note that later S&W stainless guns had carbon steel hammers and triggers that were given a flash plating with hard chrome to look stainless. These have a dull gray finish that doesn't look like stainless if you look close.
Working on the trigger grooves will break through the plating and the trigger can rust.
Ideally, the gun should be disassembled and the trigger removed to prevent any slips damaging the frame or getting abrasive dust in the action.
If done with the gun assembled tape EVERYTHING with layers of tape, leaving ONLY the front of the trigger exposed.
Tape over ALL openings into the action or cylinder assembly.
You can also cut a small hole in a plastic bag and fit it over the gun leaving just the trigger sticking out the hole, then tape the dickens out of it....Tape is cheap, damage to the gun is forever.
If you use a Dremel, buy a Cratex bonded rubber "bullet" shape 1/2 inch in diameter. These come in fine, medium, and coarse grits....use the medium, the coarse is too aggressive.
Get into a well braced comfortable position with good light and use a light touch. Be very careful not to allow the tool to "run away" and run over any part of the gun. This will leave scratches that require a shocking amount of metal removal to correct.
You can buy Cratex type rubber abrasive bullets and the arbors for them at Brownell's, direct from Cratex, and online jeweler's stores.
Buy the medium grit bullet 1/2 inch in diameter #Q15............
Cratex is a world leader in producing abrasives for many industrial uses.
For what little you need to do to make the trigger more comfortable, use the wet of dry sand cloth and a dowel.
If you use wet or dry sand cloth buy it in finer grits at automotive supply houses.
Wrap a strip around a wood dowel rod round or half round, or around a half round file, and use it like you would a file when filing metal.
In either case all you want to do is just break the sharp tops of the grooves to make it more comfortable.
Going to a full smooth trigger takes a lot of work and major metal removal, which is easy to botch up.
The main thing is to keep abrasive dust out of the action and cylinder assembly, and not to scratch or scar anything.
After the trigger is to you liking, clean everything up thoroughly and apply your favorite lube to the trigger to prevent rust.