Sift your flours to get rid of clumps, and stir your batter as little as humanly possible (no clumps means less stirring, too). Stirring incorporates air into the batter and you don't want this because they will pop and make your tortillas look like they've been in a Roger Rabbit-esq gangster shootout.
That said, you still might get air bubbles in your tortillas, especially as you near the end of the batch.This is normal, but annoying. Try dropping your bowl from about 1” onto the counter top, which encourages the air bubbles out of the batter. Or don't bother: The air bubbles in no way affect the texture of the tortillas, so it’s really no big deal.
The medium-low heat thing is serious business. Don't try to crank up the heat to crisp them up or cook them faster. That will make them pancake-like. If you're ending up with a weird texture, my first suggestion would be to make sure your heat is not too high (or your pan is not too hot).
Use as little grease as possible to get the most authentic tortilla-like texture. If you have a reliable nonstick pan - like a well-seasoned cast iron pan - use that. You may need to lightly grease it every 3 tortillas or so, and that's ok.
Don’t be surprised if the first cooked side (the one that is visible after flipping) looks like an ugly pancake.
Make sure you are cooking them until they are very crisp and feel like they'll snap in half if you try to bend them. They soften up and become pliable as they cool (and once they’re stored).