HOW TO: Upgrade The RAM to 32GB on a Mid-2011 21.5inch iMac - YouTube
How to Upgrade iMac RAM (21.5" and 27" Aluminum): EveryMac.com
Amazon.com: Timetec 8GB KIT(2x4GB) Compatible for Apple DDR3 1333MHz PC3-10600 for Mac Book Pro (Early/Late 2011 13/15/17 inch), iMac (Mid 2010, Mid/Late 2011 21.5/27 inch), Mac Mini(Mid 2011) RAM Upgrade : Electronics
Install memory in an iMac - Apple Support
Apple iMac 2011 (Mid) 21.5-inch (iMac 11,2) SSD ,Ram & OSX Upgrades - ZD Laptop Service Centre