How to upgrade RAM of MacBook Pro 15-inch, Mid 2012
macsales.com aka OWC is a fantastic website for buying RAM. I purchased 16 gb for my late 2011 model with no problems. Order the RAM amount that you want, then when the ram arrives you must unscrew the back cover of your macbook and find the RAM to remove, which you will then replace with the RAM you bought. I highly suggest watching a YouTube video on the correct procedure to do this, as it is virtually impossible to describe the process in words. Search on YouTube: installing RAM MacBook Pro. Good luck!