Heat about 1/2 inch of vegetable oil in a large skillet over medium-high heat.
Once the oil is hot, fry the corn tortillas one at a time until they are golden brown and crispy, about 1-2 minutes per side.
Use a slotted spoon to remove the tortillas from the oil and place them on a paper towel-lined plate to drain any excess oil.
Spread a layer of warmed refried beans onto each crispy tostada shell.
Add a layer of cooked, shredded chicken or beef on top of the beans.
Top with shredded lettuce, diced tomatoes, diced onions, shredded cheese, chopped fresh cilantro, and avocado slices.
Enjoy your meal!