Place chili powder, cumin, cayenne pepper, oregano, salt, black pepper and turkey legs into a 6-quart pot and add enough water to completely cover the meat, approximately 2 1/2 quarts. Cover, place over high heat and bring to a boil. Once boiling, reduce the heat to low and simmer until the meat is very tender and falling apart, 1 1/2 to 2 hours. Remove the meat from the water to a cutting board, and set aside to cool. Leave the cooking liquid in the pot. Once the turkey legs are cool enough to handle, remove the meat from the bone and shred, discarding any skin or cartilage. Place a 4-quart saucepan over medium heat and add the vegetable oil. Once shimmering, add the onion and cook, stirring occasionally, until they are semi-translucent, approximately 2 minutes. Add the garlic and chili and continue to cook for another minute. Add the meat and 1/2 cup of the reserved cooking liquid and cook until heated through and the liquid has evaporated, 2 to 3 minutes. Set aside until ready to assemble.
While the meat is cooking, place the husks in a large bowl or container and submerge completely in hot water. Soak the husks until they are soft and pliable, at least 45 minutes and up to 2 hours. If you have an electric kettle, place the husks in the kettle, fill with water and turn on. Once the kettle turns off, allow the husks to sit for 1 hour in the hot water.
Place the masa, salt, and baking powder into a large mixing bowl and combine. Add the lard and using your hands, knead together until the lard is well incorporated into the dry mixture. Gradually add enough of the reserved cooking liquid, 2 to 4 cups to create a dough that is like thick mashed potatoes. The dough should be moist but not wet. Cover the bowl with a damp towel and set aside until ready to use.
To assemble the tamales:
Remove a corn husk from the water and pat to remove excess water. Working in batches of 6, lay the husks on a towel and spread about 2 tablespoons of the dough in an even layer across the wide end of the husk to within 1/2-inch of the edges. Spoon about 2 teaspoons of the meat mixture in a line down the center of the dough. Roll the husk so the dough surrounds the meat and fold the bottom under to finish creating the tamale. Repeat until all the husks, dough and filling are used. Tie the tamales, around the center, individually or in groups of 3, with kitchen twine.
To steam the tamales:
Place a steamer basket in the bottom of an 11-quart pot and add enough water to come to the bottom of the basket. Stand the tamales close together on their folded ends and lean them in towards the center, away from the sides of the pot. Bring the water to a boil over medium heat, then cover and reduce the heat to maintain a simmer. Check the water level every 15 to 20 minutes, and add boiling water by pouring down the side of the pot, if necessary. Steam until the dough is firm and pulls away from the husk easily, 1 to 1 1/2 hours.
Serve warm. Store leftover tamales, tightly wrapped in plastic wrap, in the freezer, for up to a month. To reheat, remove the plastic wrap and steam until heated through.