Just so were clear here you are swapping out the HD for a SSD correct?
First you will encounter issues with the built-in thermal sensor and the new thin designed '12 & '13 models use a unitized power & SATA connector so you might have issues fitting the connector on to your SSD.
I would strongly recommend you look at adding the SSD via the dedicated SSD connector on the logic board Vs swapping out the HD. IFIXIT guide to replace or install a SSD unit
With all of the said here is the procedure for replacing the HD use it to take your current HD out and just use your SSD instead of the HD. IFIXIT guide to replace HD
Again, I don't think you want to do this!!
Here's a good writeup on the differences of the '12 & '13 models "Tapered Edge" Aluminum iMac Q&A
iFixit has an excellent hard drive replacement guide for your iMac at - 27" iMac Hard Drive Replacement
Anytime you open your iMac you will need to replace the adhesive foam strips - Adhesive strips
Plastic cards will come in handy. iFixit sells sets of two but old credit or gift cards work fine as well - Plastic cards
The Late 2012 iMacs also require an opening tool to initially cut the adhesive around the display - iMac opening tool
As far as hard drives go you have limited choices in the 3.5" form factor that is native to the iMac. OCZ's Vertex hard drives are available in 3.5" form factor and should work for your application. Alternately you might consider a hybrid drive, one that combines small SSD storage with large traditional platter storage. Seagate's ST2000DX001 is one such model, a 2TB traditional drive with 8GB of embedded solid state storage.
Hopefully that helps!