In a heavy-bottomed, straight-sided skillet, heat 1 inch vegetable oil over medium until it registers 350 degrees on a deep-fry thermometer. Add a corn tortilla and fry 15 seconds per side.
Fold tortilla in half, pressing one side to bottom of skillet and holding other side up with tongs. Cook until crisp and golden, 15 to 30 seconds. Flip and repeat, pressing other side into oil.
Drain on paper towels. Repeat to make as many shells as you like; a good rule of thumb is 2 tacos per person.
Store shells in an airtight container, up to 1 day.