Toasting or heating your taco shells first is totally optional. It's especially helpful if you feel the tortillas are a little stale. Heating will help refresh them. There are two methods for heating a tortilla. The first is my preference. Heat a dry skillet to medium, add a tortilla, and heat for about 30 seconds per side. If you're feeding a large crowd, this method is not very practical. Another option is to preheat your oven to 375 degrees, fold your tortillas in half and place them on a sheet pan. Heat for about 5 minutes or until warm. Overheating can dry out your shells, so watch carefully.