NFL Blitz

An onside kick can be attempted after scoring a touchdown. When in the play selection screen, place the cursor over any play, as it can be attempted after an extra point or a two point conversion. Then, hold down and . While holding these buttons, press to select the play, and the onside kick will be attempted after the extra point or two point conversion.

Recovering the onside kick is a bit luck based, but it helps to press to dive after the ball. Eventually, you will recover one.

Refer to this video for further assistance: