First, cook the ground beef. In a skillet or large pan, cook the ground beef over medium heat, breaking it up in small pieces with your cooking spoon or spatula.
Season with the taco seasoning as you cook. The meat should be cooked through with no pink remaining. Remove from heat.
On a baking sheet, assemble the tostada bases in this order: shell, a thin layer of refried beans, meat, and cheese.
Bake at 400°F for 8-10 minutes, until the cheese is very melted.Top with vegetables, more cheese (if you want) and sour cream or other condiments.
Notes
How to Make Tostada Shells (if you can't find store bought)You can make your own tostada shells by pan frying corn tortillas. Add enough vegetable oil or canola oil to a frying pan to cover/slightly submerge the tortilla. Heat over medium/high heat until the oil is hot. Then add the shell, fry for 1-2 minutes. Flip, using tongs, and cook for another 1-2 minutes. Remove to paper towels to allow excess oil to drip off. Repeat for as many shells as you need.If you want the tostada shells to be slightly shaped like a bowl, drape paper towels over inverted bowls and then remove the shells from the oil to these. The fried shells will dry over the bowls and take on this shape.