These are a sort of Oaxacan empanada or quesadilla, usually filled with refried beans, herbs and cheese. You can fill them with other things if you want. Once made they are best eaten right away. If you end up with leftovers, they can be reheated in a toaster oven or fried.
Put the masa harina in a bowl and pour about 7/8 of the cup of hot water over it. Stir it in with a fork until you get a shaggy mass, then knead it by hand for a few minutes. Divide it into 8 balls and put them in a plastic bag.
Line your tortilla press with plastic (I use a cut up produce bag) and flatten out one of the balls into a tortilla -- not as thin as with a regular tortilla for a taco, but close.
Top with some refried beans, then herbs, then cheese, leaving at least 1 inch of space around the edges. Use the plastic to fold over one corner of the soon-to-be triangle. Then fold over the next corner, patting it down to seal. Finally, fold up the bottom, again using the plastic. Pat the tetela gently to flatten it a little. Repeat with as many tetelas as will fit on your comal or cast iron pan.
Get the comal hot over medium-high heat. Set the tetelas onto the comal seam side down and press them a little with a spatula. Let them cook for a solid 5 minutes, then flip them. Cook for as long as it takes to get a little browning, about 3 to 5 more minutes. Keep the finished tetelas warm in the oven while you make the rest, or just serve as you go.