Vacuum seal the steak, getting as much air out as you can. Or use alternative methods if you don't have a sealer.
Set your water temperature bath to 120-130 for rare or 130-140 degrees. Set to cook for 1 hour.
Remove the steak from the water bath. If you wish to save the au jus from cooking for dipping, cut a small hole in one end of your bag and drain the liquid into a bowl
Heat up a pan (cast iron if you have it) or grill over high heat. Cook the steak for 1 minute per side just to get some brown on it.
Slice the steak against the grain for best texture.