I make gravey like your nana.
It's easy. Take your roast out of the pan and put it on a warmed dish to rest.
Tip most of the oil/fat off the top of the pan juices (it settles into layers so you just have to pour the top off). Add some flour to the left over pan juices and mix it all together, then add water (stirring so that it doesn't go lumpy) and bring it to a boil and let it thicken. If you have boiled any veges use that water because it is tastier than plain water. If it is too thick you can add boiling water to thin it (not cold). I season my roasts so the pan juices don't need any extra flavours but you could add seasonings. My brother chucks a couple of cloves of garlic in his roasts then pops them out of the skin and mushes them into the pan juices with the flour.
You could use corn flour but I just use plain old wheat flour. It's important to heat it with the pan juices before adding the water because if you add water before cooking the flour it tastes...... floury lol.
If I've done a roast in the slow cooker I just use a soup ladle to get the juices and make it the same way in a saucepan.

My DH always told me he hated gravy but once he tried the old fashioned stuff instead of MIL's gravox he changed his mind.