I have a different notebook but my guess is, the underside should have the same type of cover. You should best check out your manual (that should have come with the laptop) then just take out the screws of the cover where the memory chips are located (would probably expose the CPU as well).
Make sure that the laptop is turned off ok!
It would also be a good idea to ensure you buy the same type of memory chips... I am not sure if you have 2 memory chips already installed or just 1 (you will see this when you remove the cover).
One other alternative is to just bring it to ASUS and let them do it so you won't void any kind of warranty if you are not sure about the procedure.
Hope this helps!