The bottom cover has a tag with a screw under it, unscrew first then the cover will lift off, it does have little tabs around the edge so it needs care removing it. There are 2 slots of memory banks near the middle, easy to locate. You can pull out the 4Gb memory and replace with a 2 * 8Gb memory kit.
Ideally make sure the new memory kit is exactly the same brand and spec. as the memory you remove, to help with this use CPUID CPU-Z and select SPD to check the memory details of your existing memory.(more details here:
http://www.popularmemory.org/mixing-brands-of-ddr-memory/).
All memory must be 1.35v, some memory is advertised/specified as 1.35v/1.5v which can result in it running at 1.5v on a G750, ensure the memory is nailed/fixed at 1.35v.
Ensure you remove static electricity from yourself prior to removing memory and installing new memory (more details here:
http://www.wikihow.com/Remove-Static-Electricity).
Let us know if you have any further queries and how you get on.