Liver of Sulphur is the most often used oxidation method for silver including Precious Metal Clay. Liver of Sulphur is a sulphur product, so beware, it has a sulphur smell.
For beginners, I recommend buying the Liver of Sulphur already mixed in a bottle. Once you get experienced, you can buy it in a dry form and mix the Liver of Sulphur consistency yourself.
Liver of Sulphur oxidizes silver. The higher the content of copper in the silver, the faster it oxidizes. That is why sterling oxidizes (tarnishes) faster than fine silver.
You can control the speed of oxidation with temperature. If you heat the Liver of Sulphur, it will oxidize faster. Cold Liver of Sulphur, therefore, will oxidize slower.
You can also dip your silver piece in and out of the Liver of Sulphur to control the effect. Experiment with various pieces until you can control and get the finish you like, with the understanding that you will never be able to control the oxidation 100%.
If you totally hate the oxidation, heat your piece back up in your kiln, and it will come out fine silver again. Then you can start all over.
You can also control the oxidation by painting it on with a paintbrush. This works best in cracks and crevices. Whatever tool you use to dip or paint your piece with Liver of Sulphur, don't use it for anything else, because it will turn black and be unusable for any other process. I usually use a plastic fork or spoon to dip my pieces.