i live in spain, and have made dozens without any probs. i´ll just tell u my method. u basically need spuds, eggs, and onions, dice the spuds, and slice the onions, whisk the eggs, now then, i usually almost fry my onions, then parboil the spuds, after spuds and onions have been cooked, put in a bowl, and allow to cool for say 15mins, then add the eggs, and little salt, and mix gently, then put a good quality frying pan on a high heat, add oil, pour eggs, spuds, onions in ,this is the tricky part, you then have to make sure the mixture doesnt stick by gently moving the contents around pan, will need to use a dispatcher, to ensure it doesnt stick to the sides, then turn heat down, leave for about say 15-20mins, but just have a look every now and then. then when it appears settled,cooked, you will need a plate bigger than the pan, pop plate over pan, and allow tortila to fall on2 plate, might need to strike the pan, the slide back in2 pan to cook other side, it will need practice, and patience
good luck