To make homemade taco shells, you will need a taco mold, or follow the instructions given below (see Note) on how to construct your own mold with aluminum foil.
Make the guacamole: In a medium bowl, mash avocado with the back of a fork until desired consistency. Stir in lime juice and half the chopped cilantro, and season to taste with salt and pepper. Set aside.
Make the beef filling: In a large skillet over medium-high heat, heat oil. Add onion, and cook until translucent, about 10 minutes. Add sirloin, oregano, cumin, 1 teaspoon salt, and 1/4 teaspoon pepper. Cook, stirring occasionally, until meat is cooked through, about 4 minutes. Cover to keep warm, and set aside.
Make the shells: Preheat the oven to 350 degrees. Wrap 4 tortillas in aluminum foil, keeping them flat. Place foil packet and taco mold, if using, in oven for 5 minutes to heat. Remove packet and mold from oven; remove tortillas from the packet, and drape each over a taco form. Return to the oven, and cook until crisp, about 10 minutes. Remove from oven, and let sit on mold for 2 minutes until slightly cooled. Repeat with the remaining tortillas.
In separate bowls, set out the beef filling, grated cheese, shredded lettuce, guacamole, sour cream, and salsa. Give each person 2 taco shells, and have them fill their own.