It depends on you're skill level, If you have never built a system before, it could be daunting, but its (usually) not very difficult. That being said, sometimes if you have a premade computer (dell, gateway, ect) sometimes they get 'clever' with their mounting systems. Just be careful, go slow, and note where everything goes (its nearly impossible to put a wire in the wrong spot).
as a rule, if it takes a lot of force to put something in (or take it out), you are doing it wrong/it doesn't go there.
And if you ever get stuck, just google it, there are many many good guides out there. If you have a prebuilt system, you can google "how to disassemble [brand & model number]"
If you have never built/taken apart a computer before, it might be worth reading through a few guides before you start.
best of luck!
-Connor