Preheat your oven to 350°F. Place the tostada shells on a baking sheet and warm them in the oven for about 5 minutes until they are crispy.
In a medium bowl, combine the shredded chicken, chili powder, cumin, salt, and pepper. Mix well to ensure the chicken is evenly seasoned.
Spread a layer of refried beans on each tostada shell.
Top the beans with the seasoned chicken mixture.
Add shredded lettuce, diced tomatoes, and shredded cheddar cheese on top of the chicken.
Return the tostadas to the oven for an additional 5-7 minutes, or until the cheese is melted.
Remove from the oven and let cool slightly.
Top each tostada with a dollop of sour cream, slices of avocado, and a sprinkle of fresh cilantro.
Serve immediately and enjoy!