Take your biggest pan (it needs to be a whopper) and set it on the stove. Attack your bird carcass with a meat cleaver and hack it into bits that will fit in the pan. It can be just two or three big sections if your pan is big enough. Include anything that is stuck to the carcass, any leftover bits of meat or skin, the gravy and jelly it exuded onto the plate, the orange you stuffed inside it (you mean you don't stuff an orange inside your duck before roasting it? Oh you really must try it, or a lemon if you're roasting chicken). Cover it with water and turn on the heat. Now add to the pan a few vegetables to add to the stock - Just rummage about in the cupboards and see what you can find. A carrot, an onion, a leek, a couple of bits of celery, whatever you've got, but nothing starchy like potatoes or parsnips. Chop them into bits big enough to fit in the pan - we're not finely chopping veg for soup here, just hacking stuff up to make stock so don't waste time making 1/4" dice or anything. Don't even worry about peeling them or taking off the tops, just heave it all in. Add also a few whole herbs and spices such as bay leaves, a few peppercorns, springs of parsley or rosemary or whatever you've got knocking around. Again, don't chop them, just drop them in. Then leave it all bubbling away for a couple of hours.
Strain it into a bowl and save the liquor. Don't strain it into the sink, that's the whole point of the exercise. Throw out the bones and veggies and all the yukky looking stuff. The beautiful, divine-smelling, clear-coloured stock is what you've gone to all this trouble for. Believe me, Mr Oxo sells nothing like this.
Measure how much stock you have. For every pint of stock put a big dollop of duck fat into a pan (you mean you didn't save your duck fat? You fool! What did you do with it then? No, don't tell me, I can't bear to hear. Well use butter instead and next year save the fat) and then gently fry a pound of chopped mixed vegetables (one pound for every pint of stock, that is). The exact mixture isn't important, but the flavour is nicer if there is a fair bit of variety - if it's all potatoes it's rather dull. I'd want something oniony - onions, shallots, leeks, anything like that, and also perhaps some carrots, celery, swede, potatoes, nothing green like broccoli or cabbage though just because it turns the soup a nasty brown colour. Once you've sweated the veg for a while, pour the stock back on top, put a lid on and leave it to simmer for another hour or two. Then whizz it up with a stick blender (or put it laboriously through a liquidiser in batches, or rub it even more laboriously through a sieve and remember to ask Santa for a stick blender next year). Slosh in a generous slug of Madeira (which you already have in the cupboard as it's an essential ingredient in gravy) or failing that port or wine (red or white) or whatever alcohol you've got short of a Bacardi Breezer, and slosh of cream and stir it all in. Have a taste and see if you feel like adding any salt or pepper, but since it's based on fantastic stock you may well need neither. A spot of chopped parsley as you serve it tells people this soup has nothing to do with Mr Heinz. If you haven't any parsley, don't worry - as soon as they taste it, they'll know.