As above, unclip the connection on the (when your looking down at the engine bay) top left hand corner of the engine cover.
Then undo the air feed connection (bottom right of engine bay).
Pull up (hard!) take the engine cover off.
Turn it upside down on the grass or something soft (so as not to scratch the cover).
Undo the screws, theres a few, but do NOT undo them all the way, you only need to undo them all a certain amount and the airbox area (big rectangle) slides open on a hinge.
Good practice to get the hoover out whilst you've got it all open, take out the old filter and hoover up any excess dust and crap in the airbox.