This part takes the most practice.
It's easiest to do this on a clear, sunny day.
Bring the covered screen outside. Remove the black posterboard on the side of the screen flush to the metal frame, exposing the backside of the image to the sun. Whip out a stop watch and note the time. Angle the screen to the sun, cover it up after 30 seconds, and bring it over to the shadows to check how the exposure is working (see in next step).
Too much exposure and the image won't wash out. Not enough exposure and everything will wash out. Exposing is the toughest part to DIY.