First off, unless you absolutely have to use your hand, I would recommend strumming with a pick. You'll get better tone, a more even strum of the strings, and easier control of the strum.
When I do have the occasion to strum using the fingers, here's how I do it - I hold my hand straight over the strings and put all four fingers together so the fingernails make a sort of flat edge. Then I form a "J" shape with my four fingers together and strum down hitting the strings with my fingernails. On the upstroke I release the "J", or scoop, shape, and keeping my fingers together, strum upward with the tips of the fingers touching the strings.
Heh, hope that made sense. Too bad I can't just show you. Take it easy, Mart.
