Heat the vegetable oil over medium-high heat and fry the tortillas lightly (not until crispy). Add additional oil, as needed.
Remove the tortillas from the pan and drain on a paper towel lined plate.
Reduce the heat to medium and add the butter. Once the butter is melted, gently add the eggs. Cook until the whites are set, but the yolks are runny (see above for tips on frying the eggs).
Put the tortillas on a serving plate and gently slide an egg on top of each one. Cover with warmed salsa. Add your favorite garnishes.
Notes
If you like refried beans with your huevos rancheros, I recommend spreading a layer on the tortillas prior to adding the eggs.