Purchase an inflatable "blow-up" screen. You can also rent one, but if you're planning to screen several films, the investment will pay off. Look for inflatable screens online or at discount department stores. You should be able to find a 12- by 12-foot screen for about $200 as of 2011.
Clear a 20-foot-wide area of rocks, twigs or branches or sharp objects. Take the screen out of tote bag or box and place the front facing downward on the ground or lawn. You can also unfold it on top of an old sheet. Make sure there are no twists in the outside frame before plugging it in to activate blowers.
Attach the screen itself to the back of the inflatable frame. Typically, it will clip on all the way around the screen. The wrinkles from folding and storing will not be noticeable once it gets dark and the projector is shining light on it. When you are ready to go, plug it in and watch the whole thing inflate in seconds.
Use any tie-down cords and stakes to secure the screen. They clip to loops on the sides, top and bottom. Tie them around trees or fences or hammer a stake into the ground so the wind doesn't topple the screen. Have a friend hold the screen while you attach and secure the tie-down cords.