Over low, simmering, heat, add bacon grease to the skillet and allow it to warm through.
In a jar, or small mixing bowl, add in the 2 heaping tablespoons of flour and about 1 cup of the milk. Tighten lid, and shake vigorously for about 2-3 minutes or until the mixture looks free of clumps. Or, if mixing in a bowl, use a whisk and mix to make sure the mixture is smooth.
Once the milk and flour mixture is smooth and evenly mixed, pour in to the warmed bacon grease and whisk to combine.
When the milk and flour mixture is heated through with the bacon grease, add the remaining milk, salt, and pepper, and then turn the heat up to medium-high heat, constantly whisking. The gravy will begin to thicken and you'll want it to cook for at least another 5 minutes to "cook" the flour in the gravy.
Once the gravy has thickened, and has cooked through for about 5 minutes, taste test the gravy to see if you should add more salt and or pepper. Serve!
Notes
Tip to prevent clumps: If the skillet is too hot before step 2, clumps will develop. Make sure the pan is gently warm, and not too hot. If you have lumps, whisk vigorously until the clumps break up. If the clumps are too big or there are too many and the whisk isn't working, you can also use a hand blender or regular blender in a last effort, or even strain it!