You had recently installed Latex from the repo or from the upstream iso image. Now you want your own custom latex class file to be installed in your system. So how to install it? Just follow these instructions.
Create a directory called texmf in $HOME. Create a directory called tex inside an another directory called latex and then another directory with a name of your choice. So finally it should resemble something like this
* -- Directory name of your choice.
copy your .cls and the .tex files to that directory. Open the terminial and run
that's it you have installed your very own Latex class file.
To check if the *.cls file is installed run kpsewhich *.cls in the terminal as a normal user it should return the location of the *.cls file in the terminal.