The Mid 2012 MacBook Pro 13" can accept 2-8GB RAM modules for 16GB total, despite Apple saying it can only accept 8GB.
If you only want 8GB, then place 2-4GB RAM modules in, however I would just upgrade to 16GB (2 x 8GB) now and enjoy the benefit and only opening your machine once.
16GB is really sweet, allows one to open many apps at once, even other operating systems in virtual machines.
Windows in BootCamp or Virtual Machine?
Get your RAM at Crucial.com or at least use their online checker to make sure your getting the right specs.
OtherWorld computing has RAM/tools and also videos how to go about it without damaging your machine and violating your warranty/AppleCare.
Install/upgrade RAM or storage drive in Mac's