Enough liquid to cover it and usually in a pot with a lid. I've got a few large le creuset casseroles that I bought in their factory shop (and therefore affordable).
Apparently, all Agas have a different temperature in their simmering oven. I've had 2 2-oven ones - a conversion and a new gas one and now have a 4-oven electric. The simmering oven in the current one is really hot, and with something like a bolognese sauce the tomato can get a little burned and bitter if I'm not careful. So it's better to use the warming oven.
If you look on the plate by the thermometer at the top, there is a small number in one corner. Each thermometer is calibrated and there are 10 versions of the plate.
The GH Aga books are better than the Mary Berry one, and there is a brilliant little book by Catherine Blakeley - the Aga owners' cook book that you might be able to find out of print.
Our simmering oven is so hot that the Christmas cake takes under 5 hours when in Catherine's book the time is 11 hours!
Once you're used to it, you can just adapt ordinary recipes without much problem.
Have you made scotch pancakes on the simmering plate yet? The boiling plate is also really good for ageing treasure maps so that the paper's brown.