Corn Tortillas. Only corn tortillas. Fresh is better.
Cheese. We like to use mild Wisconsin cheddar in these parts, but in Mexico you would use "Chihuahua" which is similar to mild cheddar. The kind that is blended cheddar and monterrey jack is fine, as well. I think some restaurants and recipes use mozzarella these days which I think is weird and must be because it's cheaper.
Onion. White or yellow.
Spray oil like Pam or whatever you've got. You could use corn or olive oil and a brush as well.
Now, get a couple of quarter sheet pans and put them in the oven and turn the oven to about 400. Don't use insulated pans, we're using them to "fry" the tortillas. When they've been in there for a little while and are hot, open the oven and pull them out and spray the pans with the oil. Then put your tortillas on them, and then spray the top lightly.
Close the oven and wait about 3 minutes. When you look at the tortillas they should show "bubbles".
Now take them all out and stack them on a plate. You can put another batch in now, but if you do you might get distracted and let them get too cooked.
Put a large pan in front of you with a pool of sauce in the bottom.
Make sure you have sauce left over!
This is all a matter of preference, but I don't like enchiladas that are just
swimming in sauce, and if you put all your remaining red sauce on now, your tortillas will get too soft. They should have some texture and bite-ability, and not be mush.