6 or so large potatoes, slices and chopped (see picture for help)
A buttload of olive oil. Okay not that much, but a lot.
Get a pan the size you want your tortilla (probably like 10" pan) and load it with like .25-.5" of olive oil. Put all the chopped potatoes and onions in there. Cover and cook on med for 12-15 minutes until soft, stirring 2 or 3 times. The onions will get sweet and the potatoes soft.
In a large bowl crack about 6 eggs in a bowl and mix with fork. Add the cooked onions and potatoes to the eggs. (strain out the extra oil with a slotted spoon. Dump most of it, leave a tiny bit in the pan for the next step). ADD SALT and stir and let them hang out for 10-15 minutes in the bowl, it will slightly thicken. Don't for get to ADD SALT.
Put your pan on med and pour the potato & egg mess into the pan once it's hot. Cook for a few minutes until it moves easily in the pan and is a "spanish tortilla" brown. Using something flat and larger than the pan, put it over the pan and then flip the tortilla mess onto the flat surface. then quickly slide it back in the pan to cook the other side to the same doneness. Voila!
I added more olive oil after this picture below. This is NOT ENOUGH if you want to to be like those old-school Spaniard ladies that I learned this from. Drown the potatoes!