It's really easy, just start with some bacon grease and flour, enough of each so it shouldn't be dry and crumbly but shouldn't be like soup either, heat the mixture until it gets a rich brown color. then whisk in canned condensed milk + water mixture of about 50/50. regular milk will give it a sweet taste (I don't like). simmer it until thick, if it's too thin then just keep simmering it. it gets smoother the longer you simmer.
Edit: don't forget salt! and if you like some black pepper but not until near the end of cooking and never pepper the grease.
Or just use instant gravy mix lol