As digitaldoc said, use Crucial to determine the exact specs of the RAM you need.
Purchase 2x4GB sticks, with the specs you found on Crucial.
Disassembly should be pretty straightforward. You'll likely need to remove a side panel etc. Can't seem to find any videos for you.
Once you can see your motherboard, you'll see the RAM (likely only filling 1 slot). Eject that RAM (clips on the side), and seat your new RAM in both slots (be careful, and make sure they're lined up correctly - they only insert one way). Push both until they 'click'. Close up the case & boot.