You should place all themes in the sites/all/themes folder of your Drupal installation. If you have not installed additional themes yet, this folder may not exist. If this is the case, simply create a subfolder in the sites/all folder called themes.

Download the WhiteJazz theme distribution and extract the files. The WhiteJazz theme is distributed in a pkzip or a compressed tar file. Download the appropriate distribution for your platform and extract the contents using a tool such as pkZip, 7-Zip, or Stuffit. Linux users can extract the file using the tar command.

Locate the themes folder of the WhiteJazz distribution. Within this folder is a subfolder named whitejazz. This is the folder that you will copy to your Drupal themes folder.

FTP/Copy/SCP the whitejazz folder to the sites/all/themes folder of your Drupal installation.

Enable the theme by logging into the administrator account of your Drupal installation, pointing your browser to /yoursite/admin/build/themes, and checking the 'Enabled' checkbox next to the WhiteJazz theme. If you want to use WhiteJazz as the default theme for your site, select the Default button next to the WhiteJazz theme. Click the Save Configuration button to save the changes.

For detailed instructions on installing and configuring Drupal themes, check out the Drupal.org handbook page on theme installation.