Melt the butter on medium-low heat.
Slowly sprinkle the flour into the butter, and constantly whisk until you have a paste. Add the pepper halfway through. This step should take 1-2 minutes.
Slowly pour the liquid into the flour&butter mix (roux), and keep whisking. Keep whisking until perfectly smooth, and no clumps.
Keep cooking and slowly whisking until the gravy reaches the desired thickness. See #2 below