Paul Schulz wrote a step by step procedural document using power tools for his students at Lincoln-Sudbury High School for squaring a board.  Andrew Davis has submitted Paul's document "Squaring a Board" for those of us who haven't mastered the hand plane approach. 

In addition, you can click No-Fail Routines For Jointing And Planing to read the article of the same name written by Wood Magazine staff and published in Wood Magazine.  The article includes pictures and a video providing step by step instructions.  Special concerns about grain direction, sizes, and rough shapes are addressed.