How do you CFrame an object to the character and it stay there?

I'm making a horror game but want to cframe this to the character so when they zoom in they see the arm then i'm going to the make the players actual arm transparent... but I just need to know how to CFrame this object to the players left arm and it stay there as they move... so if someone could provide me with an example that would be much appreciated! :D NOTE I know using a tool would be much easier but I don't want to use a tool