Instructions
Press the "Square" button on the PS3 controller before the kickoff to bring up a list of your kickoff options.
Press the button that corresponds to the "Onside Kick" option. Depending on the team you are playing as, this button may vary.
Press the "X" button to start the onside kick power meter. Press the "X" button a second time when the power meter reaches the desired power level.
Press the "X" button a third time when the meter reaches the desired accuracy level to deliver the onside kick.