Using gravel trays

I would like some info about using gravel trays please.  What sort of gravel is used, is it mixed with sand or anything?  How deep does it have to be, and are the pots just sat on top or buried a little?  They will be used for growing Show Auriculas in clay pots.

I buy bags of pea gravel from wickes they are nicely bagged and clean to take home in my wee car. Yes bury the pots slightly just an inch or two. Fill the gravel to the top of the gravel trays or 4 inches or so

Thank you Weejenny, that is most helpful.