its easy.

just take 1/2 liter of boiling(min 60°C) water in a big cup.
put 1 teaspoon of salt in it. mix it.

take a 9V block battery put one wire(red) on the + and put the Silver-Object on the other end of the wire(red).
put another wire(black) on the - of the battery and then put BOTH wires(black first and then red) in the saltwater (not the battery ;-))

the silver is getting dark very very quick!
i let it in for about 5 seconds. i checked the side i want to darken and then i took it out of the water and wetsandet the front
to getit shiny again.

Yes the whole object is getting darker. and it is the best - cheapest and easyest way.
you can take some chemicals too. but thy are toxic expensive and dangerous.

maybe you should try an older/other pice of silver first.