This is obviously the hardest part of this guide. Fortunately for you and me, iFixit has already produced one of their professional guides to get you to this point.

Please see: iFixit's guide for removing all the internal components including the logic board

For the final step to remove the logic board, I found a flexible shaft (pic 3 - sorry it's blurry, shooting left handed) almost a necessity, as I couldn't get a straight shaft screwdriver in position and develop enough torque to loosen the screws.