Open the cover, then release single (narrow) piece of velcro. At the end where it goes into the strap there is a steel loop. Pull the velcro strap away from this still loop. This will make more of the velcro strap come out from the strap and this tightens the strap. When the straps are new, it might be a bit difficult to make them small since there is a lot of internal friction. But some "massage" of the strap (push the center of it towards the board to sort of collapse it) will help. Then stand on the board and pulling quite hard in the velcro strap. I have super narrow and thin feet and can still get them to fit (although the smaller original slick strap I which most thought were to small actually fitted me better).
For 2011 there is a totally new footstrap which has a more classic double velcro style closure.