Half-and-half water-and-milk rather than just milk, more egg than you'd think, don't open the oven too early or they'll sink. Everything else is superstition.
I used to be concerned about not getting the fat hot enough, but I've experimented and it makes little difference - it needs to be hot, but don't obsess over it. Too hot an oven - or too high a shelf - can scorch them though, when they inevitably balloon majestically. Old unwashed tins make no difference - I've used Grandma's old tins, I've used new ones from Sainsbury's, I've used a mate's loaf tin because he had nothing else - all absolutely fine.
3oz plain flour
2 eggs
2.5 oz milk
2.5 oz water
Whisk together, or use a blender. Or a fork like my Grandma did. Just make sure you've bashed the lumps out of it. Leave for hours if you like, in the fridge or at room temperature, or use as soon as the oven is hot. It honestly doesn't matter. Heat fat or oil in a tin in the oven, somewhere around Gas 6 or 7 (200°C - 220°C). Maybe hotter, but in that case use a thinner layer of batter and be prepared to hoik them out early, otherwise the outside will burn before the middle has set. I use pork fat as my cooking medium of choice, because I always save any fat from cooking pork, ham and bacon in a jar in the fridge, but vegetable oil works, so does olive oil, so does beef dripping - the flavour will be different but they all work. I've used butter successfully. Smoke point schmoke point. Pour your batter into your hot oil/fat, and bung 'em in the oven. That batter mixture will do two dinner plate sized puds (using sponge tins) or four saucer sized ones. I don't really do little ones - I'm sure you'll work it out though, if that's your thing. They'll probably take around 20 minutes, maybe a little less, maybe a little more, but I've used new fan ovens, conventional electrics and knackered old gas ovens with leaky door seals, and there's not a massive difference. Don't be suckered into thinking it needs to be kiln-like heat; super-hot does work if that's what your oven is already doing, but it needs more careful monitoring.
Trust me on the water, trust me on the eggs, trust me about not opening the door too early. I've played with all the variables, and these are the only ones which I have ever found to have had any effect.
Best of luck...