Measure butter (or oils) into your cast iron skillet.
Turn burner on stove to medium heat, and pre-warm your cast iron skillet.
If you want to use garlic, onions or shallots, prepare them now.
When pan is hot, add slices of liver (they should gently sizzle or bubble), and any alliums you prepared.
After adding liver, decrease the stove burner to medium-low (about a 3 on my electric stove).
Liberally sprinkle the surface of your liver with salt.
Cook liver for 1-2 minutes, until brown shows on the edges and creeps up through vein or artery holes.
Use a fork to flip the liver and once again sprinkle with salt.
Cook for about 1-2 minutes, until you see blood pooling on the surface.
Flip liver again. Cook it for about 1 minute, before flipping again.
Continue to flip liver every 1 minute as it cooks under low heat, until the blood no longer pools and the inside is a very faint pink (sometimes it's hard to get it perfect).