Cut the fat into small, even pieces — or score skin in a crosshatch — and heat it gently until it stops sizzling and turns from a cloudy solid into clear liquid you can pour. Ten minutes covers a strip of bacon in a skillet. Two or three hours covers a pot of cubed leaf lard in a low oven. The clock is a rough guide. The sound and the smell tell you more.
Why the heat has to stay low
Fat inside meat and skin doesn't sit loose. It's packed into cells, and those cells sit inside a mesh of connective tissue, mostly collagen, that has to soften and give way before the fat inside can escape and pool as liquid. Push the heat up too fast and the outside of the piece — skin, especially — reaches browning temperature before the tissue underneath has had time to open up. A duck breast seared hard and fast browns beautifully on top while the layer of fat just under the skin stays largely unrendered; the surface finished browning long before the structure below let go of anything.
Stopping early isn't a raw-meat safety problem — the same heat that renders the fat also cooks whatever skin or meat is attached to it. The real cost of stopping early is shelf life: fat left cloudy with trace water in it breaks down in storage much faster than fat rendered all the way through.
Pan-rendering skin and bacon
Duck skin, chicken skin, and bacon all render best in a pan you start cold or barely warm, with no added oil. Lay skin-side down and bring the heat up gradually over medium-low, aiming for something around 300°F (150°C) at the pan surface. Starting cold matters, because it gives the fat time to render out before the surface has a chance to seize and brown. Give duck or chicken skin 15 to 20 minutes, pressing it flat with a spatula now and then so it renders evenly instead of curling into a cup that holds fat rather than shedding it. Bacon runs faster: 8 to 12 minutes over that same medium-low heat, flipped once the first side has given up most of its fat.
Rendering a bulk fat: leaf lard and beef suet
Leaf lard and suet come as solid sheets or blocks rather than a thin layer, so they render on a different scale. Cut or grind the fat into pieces no bigger than an inch, so more surface area meets the heat and no piece is still solid at the core when the rest is done. Put it in a heavy pot with a splash of water — the water keeps the fat from scorching in the first few minutes before it really starts to render, and boils off later on. Hold it at a bare simmer on the stovetop, or in a 225°F to 250°F (107°C to 121°C) oven, and expect two to four hours depending on how much you're working with.
How to tell it's working
Sight, sound, and smell move together. The fat turns from a cloudy, whitish solid into clear, pale gold liquid as it renders — cloudiness means water and protein are still suspended in it. The crackling you hear at the start is water boiling out of the fat and the connective tissue around it; as that water runs out, the sound drops off and goes nearly quiet, which is the clearest sign you're close to done. Smell shifts too: clean and faintly savory through most of the process, sharp or acrid the moment it starts to scorch. Pull the pan off the heat the instant you catch that sharper smell — a few more seconds is the difference between rendered fat and burnt fat.
What to do with the cracklings, and how the fat keeps
Once the crackling slows to near silence, strain the liquid fat through a fine-mesh sieve or a layer of cheesecloth while it's still warm and pourable — it thickens fast as it cools and gets harder to push through a strainer. The solid bits left behind are cracklings: salt them while they're still hot and eat them as they are, or scatter them over greens or grits. Rendered fat that's been strained clean keeps in the refrigerator for about six months and in the freezer well past a year.