1. Download the template you would like to install. It will/must be an archive file (known as .rar and .zip files). If it is a paid template, click Buy Template and follow the instructions - after the payment you will receive the download link.
2. Once downloaded open the archive and make sure that it contains all files required for the template; tmpl.txt, at least one .css document, and an images folder. Archives may also contain other files, usually each archive will contain its own installation tutorial - these are important so read them and don't slack off.
3. Check that all required modules for the site are enabled. Your design will not be applied to modules enabled after the installation.
4. Open Control Panel > Customize Design > Template Builder. Now open tmpl.txt and copy all text from the document using CTRL+A and CTRL+C in respective order. Paste the text into Template Builder and click Save.
5. Open Control Panel > Customize Design > Style Sheet. Now open the .css file located within the archive. Copy the whole documented as in the previous step. Paste the text over the existing CSS content and click Save.
Now that we've completed the above upload the images from the archive via the File Manager or through the use of FTP