Put water and salt in a pot, cover with a lid and bring to a boil. When it's boiling, turn off the heat and remove the pot from the stove.
Immediately add in the flour. Carefully and immediately mix everything together until well combined. It's going to be a stiff dough.
Carefully put the dough in a double layer piping bags with a star nozzle inside. Immediately pipe 3 inch (8cm) of churros on a baking tray with a baking sheet. Use a scissor to cut off the dough. If the dough is too hot to handle, wait a few minutes. Don't wait too long as the dough gets hard when it's cold, making it difficult to pipe.
Note: while doing this, you can heat a pot with abundance of oil on medium heat
When the oil is hot, carefully place some churros inside and fry for 5 minutes until golden brown and fully cooked. Keep flipping them for even cooking. Don't fry too many at one time to avoid overcrowding the pot and resulting the churros to be not crispy.
Once fully cooked, remove from the pot and place on a kitchen paper to absorb the oil. Once they're cool enough to handle, you can fill them.
Filling the churros
Fill a syringe with dulce de leche. Insert the syringe at one end of the churro until the end but not poking it. Press the syringe while slowly pulling it out. Roll the filled churros in sugar and serve.
I recommend to prepare everything first before starting as some steps need to be done quite fast and having everything prepared beforehand will avoid chaos.