MsVestibule I bought a fat separator jug, but found that my old method of pouring off all but last quarter inch of fat worked better. It now lives in the dusty cupboard of frankly unused kitchen devices.
If you hover over my name you will see....
... that I am the Goddess of Gravy Making.
To make Goddess gravy.
Remove joint from roasting tin. cover in foil and a teatowel and leave to rest. The longer you leave it the better, my record is five hours. (but half an hour will do). The meat just gets better, the oven is now empty again for other stuff and you have time to fart about with gravy, veg, bread sauce, yorkshire puds, whatever.
Pour off all but quarter inch/5mm of the clear fat on the top of the yummy gunk in roasting dish. You should pour it into a jar or cup, it has a catastrophic effect on the u-bend. If it's beef, you can recycle it for roast potatoes, it is Dripping. If you've cooked pork, it's Lard. It won't be all shiny white like the purified stuff in supermarkets, but it'll do the job. Goose fat is also reusable, but I've not yet found a new purpose for chicken or lamb fat.
Put in a good sprinkling of flour, and stir it into the roasting tin and juices. The right amount is enough to suck up all the fluid but still be a bit runny. You can always add a little more but you can't take it out. (Like salt).
Now move the roasting tin to the hob, on a low setting, and stir it around with a wooden spatula. It will start to sizzle. At this point, add some hot fluid - meat stock if you've got it, or veg water if available, if not from the kettle, and stir like a loon. By cooking the flour in the gunk it is releasing gluten. Exactly the same as making white sauce. End of the Heston Blumenthal cookery as chemistry bit.
Keep adding liquid and stirring until it looks and tastes okay. Then jug it and keep warm until needed. Or nuke it if it gets too cold.
If you have a jar of Bisto Gravy Granules, your gravy is far less likely to go wrong, this is the same principle as it will not rain if you've remembered to take your umbrella.