To prepare a teriyaki flank steak beef recipe, you will first need to prepare the meat. Cut the beef into 1-inch (2.54-centimeter) cubes. Place the cubed beef in the crock pot and cover with water. Bring the water to boil, then reduce the heat to medium-low and simmer for 8 to 10 minutes, or until the steak is no longer pink. Remove the skillet from heat and add the soy sauce. Let the sauce reduce for 5 to 7 minutes. Add the red pepper flakes and stir to combine. Cook for an additional 2 to 3 minutes to thicken the mixture.