It is important to follow these steps in the order they are written. All should be good if you adhere to the steps given below.

Method using WordPress admin dashboard:

  • Login to WordPress admin -> Appearance -> choose 'Install Themes' (tab).
  • Click upload and choose
    Note: Be careful, don't upload the whole zip archive you downloaded.
  • Activate the theme and hit 'Save Settings'.
    Note: If you see a "missing stylesheet" problem, you are uploading the wrong file.
    To know more and understand how to install WordPress Themes please read Installing WordPress

Method using manual upload (via FTP):

  • Unzip the downloaded zip archive on your computer.
  • Look for file inside it and extract that theme folder separately.
  • Open FTP manager (of your choice) and upload the justshop folder to your server directly into directory  wp-content/themes/. If you are confused about the directory setup, please see uploading files in official site
  • Activate the theme and hit 'Save Settings'.

Note: Please see more detailed instructions here

Once you activate the theme, I advise you to add and activate a few recommended plugins like:

They are recommended but their installation will help you to achieve enhancements in functionality of eCommerce (eStore) and moreover make your site look visually sleek and to its optimum. If you are really not sure about installing a plugin, then please do not install it. You can always come back and do it whenever you want from Theme Options -> Install plugins :-)

Reminder: You should also setup URL link structure from Settings → Permalinks. Change as per your requirement.

Tip: Use %postname% for best SEO practice. Please click Save button to refresh permalinks (even if you do not change anything)

For further clear understanding on installing a WordPress theme, please check this video which explains in more detail: