Prepare a bowl of water to dip your hands so they stay wet while you shape your pupusas. Add around a cup of water and 1 - 2 tablespoons of vegetable oil to the bowl. Dip your hands in this bowl periodically as you shape the pupusas so that the dough does not stick to your hands. The pupusa dough will be less likely to crumble if your hands are wet when you are shaping the pupusas. You will also need to wet your hands to patch up any cracks that appear in the pupusas.
Combine the ingredients for the masa dough and mix it thoroughly. The dough should be wet enough that it can be shaped without crumbling but dry enough that it does not stick to your hands excessively when you shape it. If the dough is so wet that it sticks to your hands, add more masa 1 tbsp at a time until it becomes easier to work with. If the dough is still dry and crumbles when you try to shape it, add water 1/2 tbsp at a time until it comes together.
Seperate the dough out into smaller balls that are 1 1/2 - 2 inches in diameter.Leave a little extra dough in the bowl. You can use this dough to patch up and cracks in your pupusas.
Press your thumb into the middle of one of the balls of dough. Widen the hole made by your thumb by pinching the walls of the dough ball until the dough is in the shape of a bowl or cup.
Fill the bowl of masa dough with the beans and cheese. Do not overfill the bowl and make sure there is a rim around the top of the dough ball that is still exposed.
Start to pinch the rim of the bowl together so that it covers the beans and cheese. Continue to mould the dough so that the rim comes together and completely covers the beans and cheese.
Start to flatten the ball of dough into the shape of a pupusa. You should flatten the pupusa gradually to ensure that the pupusa does not crack around the edges. Start by pressing down on the center of the pupusa and continue to work your way outwards. You have two options when you flatten the ball of dough. This first is to hold the ball between your thumb and your fingers and to use both hands so pinch the pupusa until it is flat. The second option is to put the ball on a layer of sheet on parchment paper and press down on it from above.
Preheat a flat pan, comal, or skillet over medium to medium high heat and add a little vegetable oil. Cook the pupusas for 3 - 5 minutes or until they start to brown.
Flip the pupusas over with a spatula and cook them on the other side for 3 - 5 minutes.