Uploading files
Extract zip file containing style.
After extracting zip file, you will find a folder that contains all style files. Upload it via FTP using an FTP client such as Filezilla to directory "styles" of your forum.
You can verify if you have uploaded files correctly by checking path to style.cfg; it must be styles/{style_name}/style.cfg (replace "{style_name}" with style directory).
Sometimes extracted style contains additional directory created by software you used to extract style. Make sure you do not upload that directory, but upload its contents. Path to style.cfg must be styles/{style_name}/style.cfg, not styles/{style_name}/{style_name}/style.cfg
Installing style
After uploading all files, open your forum in browser, log into admin control panel.
Click "CUSTOMISE" tab and you will see list of installed styles.
Below that list you will see list of styles available for installation. Click "Install Styles" link for style you want to install.
Submit form. If you want to set new style as default forum style, enable "make default style" option.