The blackout method mentioned by Mmiller2001 is simply leaving the light off and covering all sides and top of the tank to block out all light; this includes ambient lighting. People often use cardboard boxes/paper, large trash bag pulled over the tank, etc. Photo-period is referring to how long you leave your aquarium light on for. So the suggestions are either a three day blackout, leaving the light on for 5 hours max per day for a couple weeks, or a combination of both. eg. blackout > 5 hours/day. Without a timer its a bit cumbersome, but sticking to a more consistent lighting period will help with the green water algae and prevent other algae issues in the future.