Close the lid, select the steam function, and press start. Whatever you do, DO NOT WALK AWAY! If you walk away, your cooker will continue with the steaming process and you will have runny, lumpy, custard soup instead of the lovely vanilla-scented pudding-esque solid you're after. You do not want custard soup. All it is good for is mixing with pre-cooked rice and baking as a dessert and even that is not really very good.
Instead, stay there with your rice cooker until you hear faint bubbly-type sounds. This sound indicates that the heating element beneath your bowl is finally up to temp to actually cook something. Once this happens, hit the off button and then put your rice cooker on its KEEP WARM setting. Keep warm does not have enough heating power to actually cook our custard, but it does keep the heating element engaged enough to finish cooking our custard once it has been thoroughly heated via the STEAM setting. Now, whatever you do, DO NOT OPEN THE RICE COOKER. This allows all the heat and steam to escape right out the top of your bowl. You don't want that. If you do that then you have to close it, set it back to steam, stand there again until you hear the bubbling, turn it off, and put it back on Keep Warm. It doesn't have any negative impact upon your custard to do so, it's just bloody boring standing there again for 12 minutes while the contraption reheats itself.
Once Keep Warm has been engaged you can walk away to wherever you fancy. Go do some laundry. Play with the dog. Take care of clean dishes. Covertly harvest yourself a bouquet from your neighbor's daffodil bed. Whatever. Just leave the custard undisturbed in the cooker for 30 minutes to an hour. Or longer if you're forgetful like me. Check on it every 30 minutes or so (yes, I did just say not to open the lid, but you have to go through the steam/off/keep warm dance every so often to ensure things are moving along appropriately) to check the consistency of your custard. It almost goes without saying that the less the volume of custard you are making at once, the shorter the cooking time. I am making up a LOT of custard this round. Usually I only use 2-3 cups of milk so this is like a double batch. As such, it is kind of an experiment in a way and it may fail horribly due to the extra volume. I hope it doesn't.