This is the easiest step, 4 screws and you're done!
I chose to use a Sunon MagLev Dual 40MM Case Fan KDE1204PFV2 because I happened to have it lying around...
Allign your fan with the part of the case where you removed the plastic from the inside,put it up against the 6th piece of plastic which you did not remove. Stickers facing inwards.
Allign the screw holes with the vents to make things simple just put the screws into the vent holes, or if you're feeling ambitious you can drill your own.