First thing I will say: Do not change the ownership of
/usr. You will cause masses of problems. For instance, if you change the ownership of
/usr from root, then
sudo will no longer work properly (in fact, it could render the computer unbootable from what I have heard). Indeed, there's very little that's owned by
root by default that I would ever advocate changing the ownership of - once you do something like that recursively it becomes incredibly difficult to undo it.
If you need to create a file within one of them, use
sudo. So, if you had instructions to install a program under
/opt, in a folder called
/opt/myprogram that needed to belong to you, you would perform the following commands;
sudo mkdir myprogram;
sudo chown $USER myprogram.
But as I say, for the sake of your system's stability, ONLY change the ownership on files or folders you yourself have created. If you need to edit something owned by
sudo ahead of the command rather than taking ownership.