read main article in here:
http://boards.cannabis.com/showthread.php?t=82280
several quick suggestions of my about SCROG:
*your plant goes under screen right before 12/12 when the plant is fully matured.
*u bend and and tie branches every 3-4 days for the first 2 weeks (plant usually accelerates it's growth for a first 2 flowering weeks, and then it slows down), till the beggining of whats gonna be call BUDs later starts appearing.
*try to spread future buds evenly on the surface of the screen.
*If there are leaves on your way, try to put them under the screen or just cut em' (thats what I do)


theres a pic of todays situation on one of my plants. Northern Lights 4 weeks into flowering. as u can see, all buds looks pretty much the same size, they are in the same level. I would say this is main principle of SCROG
I put my little painting of SCROG scheme too
good luck