You open both flaps and place the film in the proper taking spot. Feel to make sure the film is centered over the opening side to side then carefully close the inner most flap followed by the outer flap. I haven't had any light leaks or scratches. The pressure plate should hold the film in place, just try not to bounce it around too much. You will get an image that is 3"X4" rather than 4"X5". I think if you went with the mod they open it up to get closer to 4"X5".
I put a cheat sheet on the back of my camera for the EV values:
EV 10 = f/8.8 at 1/12
EV 11 = f/8.8 at 1/25
EV 12 = f/8.8 at 1/50
EV 13 = f/8.8 at 1/100
EV 14 = f/12.5 at 1/100
EV 15 = f/17.5 at 1/100
EV 16 = f/25 at 1/100
EV 17 = f/35 at 1/100