This is a topic I wanted to avoid.. but as I have seen a lot of shattered plates brought to me I want to give you the best advice if you couldn't find a broken ceramic and need to break one for our workshop.
One way is to use a side of a hammer like this. If you use the round head you might make a circular hole in your plate. Hold the plate on it's side against a hard surface and wrap it in some paper, plastic or a towel.
Another thing to take note of is to hit the side or an angled part of the vessel.
My friend Steve shows us using a ladle in these two videos here.
Video 1 Steve hits the side of this plate.
Every ceramic is different in hardness depending on thickness and type of clay. Ideally you want to break it on the first hit because it's more likely to get micro fractures and break in to a lot more fragments if you have to hit it multiple times. It took Steve a few times to break this one and you can see it got a little more broken then we wanted it to.
Unfortunately with things like these it's hard to know how hard to hit unless you break the same kind of plate multiple times in a row and get a feel for it.
You might need to try this on a few different plates until you get the right kind of break (ideally no more than 4 pieces).
This video below is an example of breaking something that already has cracks in it. You don't always need to break things with cracks, only if you feel the structure has been compromised.
I hope you find these useful!