[Cr3aTor]

The usage on RATs depends on how the interface of the RAT is designed, but the functionality of many is the same. Thus the interface is very similar (Keylog, make screenshots, steal passwords, etc). I will assume that "by setup a RAT" you mean spreading it. This can be done with the help of SE or other methods. You can always improvise and think of your own methods as well.
Since the RATs from the web have only a limited functionality and you can't customize it, I would recommend to create your own instead. Learn C++, WinAPI, look at how crypters work and other methods to bypass AVs, or even make your own?. Also think of what your RAT need to do (Inject other files?, Harvest passwords?, etc). Then, implement it based on your needs. I know it's more complicated than downloading some program made by others, but this way is more reliable, it matches your target, you can also add new functionality, you can make it more stealthy and a lot of other.