Start by ensuring your chicken is cooked (e.g. leftover, rotisserie, or pre-cooked). Dice or shred it, then toss with the BBQ sauce so it’s well coated and flavorful.
Preheat your oven to about 400 °F (or ~200 °C). Spread a layer of refried beans over each tostada shell. Sprinkle shredded cheese atop the beans, then arrange the BBQ chicken mixture over the cheese.
Put the assembled tostadas onto the baking sheet, then bake for 8–10 minutes (or until the cheese melts and the edges get crisp). Once out of the oven, top with lettuce, pico de gallo, and a drizzle of BBQ sauce and sour cream before serving