How To Warm Tortillas
In Mexico, they use a special flat skillet called a comal to warm tortillas. You can use any shallow skillet. One with a non-stick or seasoned cast iron surface will work best.
Put a dry skillet (non-stick or seasoned cast iron works best) on the stove over medium-high heat.
Tortilla warmer
If you have a tortilla warmer, lay a clean cloth or paper towel in it. If you don't have one, aluminum foil works well: lay a large-ish sheet of aluminum foil on a clean surface, and lay a large paper towel on top of it. Either way, you will put the hot tortillas on the paper towel, then cover them with the lid of the tortilla warmer or with the aluminum foil.
Comal
When your skillet is hot, carefully lay one dry tortilla in it. As it warms, move the tortilla around a bit in the pan to be sure it doesn't stick.
After about 20-30 seconds (possibly a bit more or less depending on the heat of your pan), you will see steam escaping from the tortilla, and you may notice a few bubbles gently forming from underneath. This means it's ready to flip.
Use your tongs (Mexicans often just use their fingers - carefully!!) to flip the tortilla over to the other side and allow it to warm for another 15-25 seconds. Again, you will see steam escaping and perhaps a few bubbles gently trying to form. As you gain more experience, you will notice other visual and aromatic cues that will tell you when it's ready.
When properly heated, the tortilla will be hot, soft and pliable, but with little or no browning from the pan.
Lay the hot tortilla on the cloth or paper towel and immediately cover it with the lid of the tortilla warmer or with aluminum foil while you heat additional tortillas. Each time you add another to the stack, cover them quickly so they stay hot while you work. The cloth or paper towel absorbs the moisture as the tortillas continue to steam, so that they don't get soggy while they are covered.