Add four to six tortillas to a baking sheet; if you need to prepare more, work in batches. I recommend one baking sheet in the oven at a time.
Brush oil onto tortillas evenly on both sides. This will ensure even crisping.
Optional: Use a salt shaker to sprinkle a little salt onto both sides of the corn tortilla.
Bake for 3-4 minutes, using tongs, flip them, and bake for 2-4 minutes longer. Check them every two minutes if this is your first time making them as they will go from crisp to burnt very quickly.
Tostadas are done when the corn tortilla will have the appearance and texture of a tortilla chip: crispy and golden brown.
Once the tostadas are completely cooled, they can be stored in a zip-type bag for several days in a cool area. I add a paper towel to the inside of the bag to absorb any moisture.
Notes
If you are using a dark baking sheet, reduce the baking time by 25 degrees. Dark baking sheets speed up baking.