Dissolve the bouillon cubes in water. We've just made stock. If we are on the next level, using original roast meat drippings, filter the drippings through the sieve to remove bits of things if desired then mix it with water.
Melt butter or use any cooking oil then mix in the flour until a uniform texture with no lumps and white specks is achieved. If it gets a slight golden brown color, it's even better, that is roux, remove from heat and let it cool a bit, about 5 minutes.
Pour in the dissolved bouillon cubes, stock or roast meat dripping.
Put it back on the heat and stir or better whisk until a lump free uniform, thick texture is achieved. Adding more water will thin the sauce which may be required especially when reheated as the sauce thickens as water evaporates and cools off.