Get into playing position. Position your pick on the top side of your biggest string - the sixth string - and slowly, one at a time play six downstrokes across every string from your sixth to your first. Don't pause after every note - keep the pick moving until it's played every string. Try to keep the loudness and the rhythm even. When you've finished playing all six strings, then reverse the angle of the pick by using a swiveling motion, much like turning a doorknob, and play the same thing as a series of six upstrokes. You have just strummed the guitar down then up, but in slow motion.