Picking is basically the most important skill in the art of guitar, whether classical electric or acoustic. The picking stokes makes the song u are playing flow. But it all depends on the song. As they said earilier on, there is down up down up. But as u may knnow, a downstroke is a heavier sound, so it is sort of the heavier accented beat, resulting is the upstroke, which is the continuation of the first beat. In the end, u get a constant row of notes, with a smooth melody line. Practice makes perfect, do scales with updownmotion of the pick, going the complete 4 octaves, or u can also do chromatic scales. Scales train note accuracy and chromatics train speed... Practice is the crux of the whole process.