I make what we call either cowboy gravy or some people call red eye gravy.
I usually make it with pan drippings from a slow roasted chuck or rump roast.Of course the roast had potatoes, onion and carrots roasted with it also. To the pan drippings that have cooled some. I add some bullion either cubed or crumbled, then some flour and some leftover coffee. Then I bring to a slow boil constantly stirring until it all is boiling as I stir it then remove from heat. As for portions it is usually about 1/2 cup pan drippings, enough bullion to mix with the left over coffee or water for a cup of liquid and probably about 2 1/2 hands full of flour. Salt and pepper to taste.
Experiment it will come to you with practice. It's fun even eating the mistakes on fresh biscuits or roasted potatoes. Good luck and happy eating!