You have two options. First, if you have a gas stove, turn a burner to medium-low and use tongs to place the tortilla directly over the flame, 10 to 20 seconds per side until charred, in spots. Or, you can add each tortilla to a cast iron skillet over medium-high heat for 15 to 30 seconds per side. Stack and wrap charred tortillas in a dish towel to keep them warm.