Cook your sausage over medium-high heat, breaking it up into crumbles/clumps.
Once the sausage is cooked through, and the outside is slightly crispy remove sausage from pan, leaving as much grease as possible in the bottom of the pan. This is important because the grease is the base for your roux, basically the essence of the gravy.
Sprinkle the flour over the grease and mix until all grease is absorbed.
Cook this paste to brown the flour/grease mixture. You want it to turn golden brown, but be careful not to burn it.
Slowly add the milk, a little at a time, mixing it into the flour mixture after every addition.
Once all of the milk is in add the black pepper and all of the cooked sausage back into the pan. Continue stirring and bring to a low simmer to thicken. Taste and add salt if desired.