Hi there - I actually haven't made it for yonks, so this is the method rather than an actual recipe, IYSWIM.
It does involve a bit of trial and error and the beginning, as the ratio of water to oats very much depends on the kind of oats you use: chunky ones will require more, fine ones less.
Because they'll be left soaking overnight (which incidentally makes them easier to digest and better for you), you may find you need slightly more liquid than it says on the packet (I'd start off using that as a guide).
So, place your oats and liquid in a heatproof bowl, that will comfortably fit inside your IP insert (not on small, the oats and milk tend to froth up during cooking). I use about a third milk and the rest water (and have no qualms about milk staying out overnight - my kitchen is cool, and if you're in the UK you should be fine) . Add a pinch of salt, and at this point I also add a teaspoon of coconut oil and a teaspoon of jaggery (hard unrefined sugar cane juice - amazing rich flavour).
Place wire trivet in base of IP insert, pour a couple of cups of water in, place your bowl of oats & liquid on top of the wire trivet. Using Manual, set the number of cooking minutes - again, this will require trial and error, depending on your oats and amount of water, and your preferred texture. I'd start with 3/4 mins. Then use the timer button to delay the start of cooking for however many hours suits you. Make sure that the toggle is set to Sealing.
You need to allow time for the pressure to drop via Natural Release, so I'd aim for cooking to finish a minimum of 20 minutes before you wanted to open the pot. Be prepared for it to look a bit wet and unappetising when you first take the lid off; just give it a very thorough stir. If it still seems more liquidy than you like, you can leave it with the lid off for a few mins:it will continue to thicken up the longer it sits.
It probably won't be perfect the first time (tho you might get lucky), so have a back-up breakfast on hand! But it is worth persevering until you find the right ratio/timings for YOUR perfect porridge, as it's just such a help to have hot porridge waiting for you (and subsequent family members at later times) in the morning.
P.S. Don't forget to put the lid back on if you're going to leave it on Keep Warm for hours (which you can, with no issues); the water at the base does evaporate slowly, and you don't want to end up with a dry pot-bottom.
P.P.S. I add a thin layer of milk (poured gently over the back of a spoon) to the surface of the porridge if I know it's going to sit around for a while waiting for the next person - this helps to prevent a skin forming on top.
Sorry this is soooo long - hope it helps!