I can only say how I do it. I take my meat juices and skim the surface to remove any fat. I put the meat juices in a saucepan and save about 1/2 a cup out in a separate dish. If the 1/2 cup is still hot I add an ice cube to cool it. I mix about 1 tbsp. flour with 1 tbsp. corn starch together with salt and pepper. Make sure all the lumps are out of the dry mixture and whisk it into the cool meat juices. (The amount of flour/corn starch will vary depending on how much gravy you are making.)
Heat up the meat juices in the saucepan and whisk in the 1/2 cup mixture, stirring until it thickens.