Plain flour
Egg(s)
Milk
Some people use salt and pepper, some salt, some neither. If you use it just a pinch.
Metal tin - traditional YP is made in one large roasting pan - often the one used to roast the meat - the meat was taken out to rest.
Whisk
Animal fat, lard or dripping or even goose fat - it gets hotter than oil
In a jug put about 1/2 a pint of milk (275ml) and then crack one or two eggs in. Traditionally only 1 egg is used but more eggs make the puddings rise more (James Martin, who I love most of the time, is a disgrace to Yorkshire as he uses 5 eggs)
Whisk the milk and egg then start adding flour a spoon at a time and whisk after every spoonful.
Once the mixture has the consistency of double cream it's done and you can leave it to rest. Tradition has it on the doorstep over night but 30mins to 1 hour should do.
Put animal fat in the tin and put in the hottest part of the oven, for a gas oven this is usually the top.
The fat needs 20-30 mins until it is 'smoking', so very very hot. Once the fat is hot you are going to put the batter in, in yet another tradition turn on the hob and put the tin on the hob - this stops it cooling down.
Pour the batter in as quickly as possible and return to the oven. Your pudding should be ready in 20mins.
Your dh is right, they don't all have that 'sink in the middle', you get that in the frozen ones you buy frozen but home made ones are often a range of shapes.
Pic of some of mine - not remotely looking like the ones you buy frozen.