To begin, bring the water, salt, and ¼ cup vegetable oil to a low boil in a medium saucepan. Once boiling, remove the mixture from heat and pour into a mixing bowl with the flour. Mix until well-combined, then let the dough cool for about 15 minutes.
In the meantime, make the cinnamon sugar coating by combining the sugar and cinnamon in a shallow bowl or plate. Set aside.
Once it has cooled, transfer the dough to a churrera or piping bag with a large star-tip attachment. Let it cool for another 15-20 minutes in your fridge. Then, bring 5-6 cups of vegetable oil (enough to fully submerge the churros) to 350-375°F in a deep pot.
It's ideal to use a thermometer when working with oil. If you don't have one, test if the oil is ready by dropping a small piece of dough in. If it sizzles and floats to the top, then you should be good to go.
Squeeze 2-3 pieces of dough out at a time over the pot of oil (around 4-6 inches long). Cut each piece with a pair of scissors or a knife. Note: don't cook more than 3 at a time since the oil temperature will drop too much.
Fry the churros until golden brown, about 2-4 minutes per batch. Using a slotted spoon, remove each one from the oil onto a paper towel-lined plate. Leave them for about 30-60 seconds before rolling in the cinnamon sugar mixture.