Place ground beef in a skillet over medium heat, and sprinkle about 3/4 of the packet of taco seasoning over the meat. Cook and stir ground beef, breaking it up as it cooks, until beef is browned and crumbly, 10 to 15 minutes. Drain excess grease.

Mash refried beans in a small saucepan; add a little water as needed to loosen (you may not need to add any, depending on the consistency of the beans) and sprinkle with the remaining seasoning. Heat the refried beans over low heat until warmed through and smooth.

Preheat the oven to 300 degrees F (150 degrees C). Place the crisp taco shells on a baking sheet, and warm them in the preheated oven for 3 to 5 minutes.

To make the guacamole: Peel, seed, and mash the avocados in a bowl with 1 tablespoon of sour cream, the lime juice, and salt and pepper to taste, and set aside.

To build the tacos: Spread each flour tortilla with about 2 tablespoons of heated refried beans. Wrap the tortilla gently around a crisp tortilla shell. Spread about 2 tablespoons of ground beef along the bottom of the crisp shell, and sprinkle about 2 tablespoons of shredded Cheddar cheese on top of the meat. Top the cheese with shredded lettuce, a sprinkle of chopped tomato and onion, and a dollop of sour cream and guacamole.