In your WordPress dashboard, go to Appearance > Themes. At the top, click Add New, and then Upload Theme on the next page.
Click Choose File. Locate the magnolia-theme.zip file that came with your theme and upload it. (Note: your theme may have a version number behind it, such as magnolia-theme.2.1.zip. The child theme file will always be all lowercase.)
Click Install Now and wait for the theme to install. Activate the theme.
If you received a "missing style.css stylesheet" error, you've uploaded the incorrect file.
Click here for further details.