Cut the flank steak against the grain into strips that are a half an inch (1 cm) thick and a couple inches (5 cm) long.
Once the steak is cut, heat the oil on medium heat in a large skillet.
One the oil is hot, add the beef strips to the skillet in a single layer and cook for around a minute and a half on each side. If all of the beef doesn't fit into the pan at one time, cook as much as will fit in a single layer at a time. When each round of beef is done cooking, remove and place aside until all the remaining beef is done cooking.
Lightly season with salt while the steak pieces are cooking.
Once all the beef has cooked, add 1/2 cup of teriyaki sauce to the pan along with all the cooked beef. Stir the beef and sauce together until the sauce reaches your desired thickness and coats all the pieces of beef.
I typically serve beef teriyaki over a bed of rice and garnish with sesame seeds and chopped scallions. Enjoy!