Slots 1 and 3 are one memory channel. Slots 2 and 4 are the other channel. Your 16GB should be installed in either one of those pairs already. So it's just a matter of putting the new memory in the available slots and hope the voltage and timings match up.
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Asus ROG G55VW:
CPU: i7 3630QM
GFX: Geforce GTX660M with 2GB ram
RAM: 2x8Gb 2133MHz Kingston HyperX Impact (surprisingly the G55VW uses the 2133 X.M.P. profile)
Storage: RAID0 with 2x 256Gb Crucial SSDs (both msata, one in a sata adapter)
OS: Windows 10 x64