How do I install the class?

The documentation of the class is the authoritative source for the installation instructions. The home page of this site offers a summary.

There are three possible scenarios for the installation of cjs-rcs-article:

System installation through your TeX distribution. This is strongly recommended. The class is distributed as the package cjs-rcs-article and is included in the main TeX distributions. If a newer version is available, simply update your TeX distribution.
Local installation limited to your project. You may prefer this option for reproducibility reasons, or you may not be in a position to update the TeX distribution if you’re using an online editor. Download the archive and uncompress it in the directory of your project.
Manual installation. TeXperts may prefer this option. Download the package cjs-rcs-article from CTAN and follow the instructions in the README.md file (this involves compiling cjs-rcs-article.ins).