Now you need to buy something that will burn. You want a substance that will burn faster and darker than the tortilla will.  In other words, you want the tortilla to get nice and golden brown, while the image gets all burned and black.   So while you are at the grocery store, pick up some browning & seasoning sauce.  I use Kitchen Bouquet brand, for whatever that's worth.  It's not like I sought this stuff out.  It's just what they carry at the store.  You will paint your image of Jesus or whatever on the tortilla with this browning sauce.  

I have used a variety of different mixes to get just the right burned look.  Molasses, brown sugar mixed with water, straight browning sauce.  If it has sugar or caramel in it, it will burn.  You should experiment with different mixes.  But generally, the more sugar is in the stuff, the shinier it gets when it burns.  And I don't like shiny.  It doesn't look like a natural miracle.  Sure, it's burns dark, but it also looks obviously painted on.  I want something with more of a natural, matte finish. 

That's why I mix with acrylic paint with the browning sauce.  So your next stop is the art supply or crafts store.  Get a color called "burnt umber."  I use Golden brand paint, but the brand doesn't really matter.  Anything will work.  It just has to be burnt umber.  Anyway, I find that adding the paint thickens the browning mix, which makes it a bit easier to spread on the tortilla, and it deadens the shininess, which makes the burned bits look more natural.

And while you are there, get yourself some cheap-ass brushes, too.  Not too big.  One of those beginner sets of five or six for $2.49 should do it.  Be sure to get ones with stiffer bristles, like in the picture here.  Don't get those soft watercolor ones.