Set your sous vide water bath to 138 degrees - the temperature for medium rare.
Add salt and pepper to both sides of the steak to your tastes.
Seal the steak with a vacuum sealer or the water displacement method.
Add to water bath. Find that it's floating? Clip it to the side of the water bath or use a heavy object like a pot to weigh it down. Make sure to not pass the max fill line for your machine.
Cook for 1-2 hours.
Remove the steak from the water bath. I use tongs so I don't touch the hot water.
You can drain the liquid inside the bag into a small bowl for dipping. It's optional.
Put a cast iron skillet over high heat. Once the skillet is hot, add 2 tablespoons of your favorite cooking oil. Cook the steak for 1 minute per side to brown each side.