Hi, I believe there are MANY tutorials on the web (including videos) about "how to" for this type of upgrade. OWC made some step-by-step instructions (for 2012 and 2013 21.5 inch iMac, the steps are almost identical for replacing HDD to 2.5 inch SSD).
There are many discussion about which SSD to choose -- I would suggest you to do some research. Again, the risk in on you.
I got my Apple OEM SSD (2.5 inch, 512 GB) from e-bay (my own risk as well), and no one provide installation guide from all SSD vendors per my knowledge. In addition to opening tools (small pizza cutter like), re-seal tapes, suction cups, and a pair of steady hands, you need to give yourself plenty of time to finish the tasks. Remember, back up is always the best principle when doing anything to a computer.
The end result -- was very rewarding, but the procedure is not for casual DIY-ers.
Do your homework and decide which way you prefer.