The construction of the enchiladas is pretty standard.
1) Buy some good-quality corn tortillas, a 10-pack if you're doing one pan, and 20 if you're doing 2.
2) Grab your standard pyrex baking dish (or metal baking pan or aluminum foil pan) and coat with non-stick spray.
3) Heat up your enchilada gravy to warm in one shallow sauce pan, and heat up some vegetable or oil in another-- enough to coat the bottom and deep enough that your entire tortilla can submerge. You're going to very lightly fry your corn tortillas in the oil, and each tortilla will pick up some of the oil, so be ready to add more if necessary.
4) Grab TWO sets of tongs. The first is for the oil. The second is for the sauce. If you use only one set, and some of that sauce slides into the hot oil, it's going to splatter all over.
5) Get your assembly line ready, it should look like this: Hot oil, hot chile gravy, warmed filling, shredded cheese of your choice. For the BB duck I wanted a mild cheese that wouldn't overwhelm the duck, so I used Monterrey jack. For my brisket enchiladas I like to use a medium cheddar, it's a stronger cheese but the brisket holds up to it quite well.
6) Take your first tortilla with your first set of tongs, dip into the hot oil and fry for a few seconds, then flip it over and fry the other side. You're not trying to make a hard tostada shell here, actually the opposite. Lightly frying actually makes the corn tortilla more pliable without breakage, and it also makes it stiff enough that it won't fall apart. With that same set of tongs, remove the tortilla and let it drain well. Then drop it into the warmed chile gravy.
7) With your second set of tongs, get both sides of the tortilla lightly coated with the gravy, then move to the pan and place in an open u-shape.
8) Place a small spoonful of your filling along the bottom of the tortilla, then sprinkle in some of your shredded cheese.
9) Roll the tortilla until it's completely closed, with the seam side down. Your hands might get a little messy. That's okay, you're allowed to lick the tasty chile gravy or meat filling or cheese-- but you better wash your hands after that!
10) Continue the process until the baking dish is full. Then cover everything with more of the gravy and plenty of the cheese. It's also common to add finely chopped white onion to the top, but I always ask my guests to make sure they like that. I'm a big fan of the onion so if it's just for me, I always add them.
11) Toss that baking dish into the oven at 325-350 until all of the cheese is bubbly and melty. Pull out, and serve.