Like I said start the car,open all the vents,make sure the air is set to recirculate,set the AC to max cold,set the thing off in the back of the car,leave it 10 minutes for the can to empty and circulate around the system then leave the doors open to vent the excess.
http://www.halfords.com/webapp/wcs/stores/servlet/product_storeId_10001_catalogId_10151_productId_224996_langId_-1_categoryId_165632