Gravy is easy to make. Here's the standard for a medium gravy:
2 tbsp oil or butter, 2 tbsp flour, 1 cup liquid
or
4 tbsp oil or butter, 4 tbsp flour, 2 cups liquid
You just cook equal amounts of oil, fat or butter and flour then add a liquid. You can use vegetable oil, olive oil, fat or grease that has cooked out of meats, lard, butter, shortening, etc. For the liquid, you can use water, vegetable broth/stock, chicken broth/stock, beef broth/stock, base and water, bouillon and water, or any flavorful liquid. Milk is used for white gravy, sawmill gravy, sausage gravy for biscuits, etc.
For brown gravy, heat 2 tbsp of oil or butter in a skillet and stir in 2 tbsp flour. Cook it, stirring constantly, until it is thoroughly mixed and browns, about 2 minutes. The mixture should turn brown, but do not let it scorch or burn. If you use butter, cook at a lower heat because butter will burn faster than oil. This oil/flour mixture is called a roux.
After the mixture has browned, add 1 cup of liquid and stir constantly until mixture thickens. Season with salt and pepper, to taste. That's it! LOL
For a white gravy, which is what I make for biscuits, you only cook the oil/flour mixture about a minute, not long enough to let it brown. Then add milk.
If you use bouillon (cubes, granules or base) mixed with a cup of water for your liquid, you can add a little more than 1 cube or 1 tsp for more flavor.
If you have cooked meats like ham, ground beef, bacon, a roast, fried chicken, fried pork chops, etc., drain all but 2 tbsp of the fat left in the pan, stir in flour and scrape up all the cooked bits from the bottom of the pan for great flavor.
If you want a thin gravy, use 1 tbsp oil or butter and 1 tbsp flour and 1 cup of liquid. If you want a very thick gravy, use either 3 or 4 tbsp of each with 1 cup of liquid. I like a medium gravy best.