Here's a step by step photo tutorial from PW: http://thepioneerwoman.com/tasty-kitchen-blog/2010/01/how-to-chop-an-onion/

First cut the shoot end off, then cut in half through the root. Lay cut side down and make a series of horizontal cuts through the onion, then make vertical cuts according to how large of a dice you want. Finally, chop the onion perpendicular to the vertical cuts.