CMSimple Templates are mostly available for download as .zip files. Unzip the file with a suitable program. A CMSimple template at least contains template.htm, stylesheet.css and a folder images/.
These files and subdirectories of the template (and possibly more) you will find after unpacking in a folder with the name of the template.
Upload the folder with the name of the template (and the subfolders and files described above) to the folder ./templates/ in the installation directory of CMSimple.
The following files must be writable (chmod 666):
For some templates, additional files or folders must be given write access, usually there is somewhere in the download a readme.txt or something in the way with more information.