Here is a how to video on installing a Cabin Filter on a 2012 Chevy Equinox. This install should be the same for 2010-2013 Chevy Equinox's and GMC Terrain.
I could not find much info or pics regarding the install, I have searched the forum and found a few directions, some even said it was a PITA and it can be because of the angle of getting to it, but I hope this video will help answer your questions.
Or try this link
This filter retails for about $30, but I bought it from Amazon since it was the cheapest there at $15 and I got free shipping since i also ordered the engine air filter at the same time. You can find this part anywhere online that suits you best. Your Dealership will sell it for around $30 or more which is a little much and plus they will charge you to install. But as you see in the video it's easy!
GM Part # 19130294
AC Delco# CF177
Here are a few other brands:
FRAM fresh breeze #CF10742 around $25
Wix # 49368
I have seen some people make their own cabin filter probably for much cheaper, and may even filter out better if they have severe allergies. Some of those home pad filters they buy for $5-$10 , some people get 4 filters out of those pads. I didn't want to mess with that , or cut to match the stock size filter, I just ended up using the stock filter since i know it works and will fit properly.
Hope the info helps!