Installing the WordPress child theme is done in the same way as installing any other WordPress theme. This however has to be done after installing the parent theme.
To install the child theme, you will need to:
1. Navigate to the Downloads page (themeforest/downloads)
2. Click the ‘Download’ button next to your item and select ‘All files and Documentation’
3. Extract the .zip file to your hard drive
4. Navigate to the folder containing the child theme zip file
5. Inside your WordPress admin area, head over to Appearance > Themes page and click on the Add New button
6. On the next screen, click on the “upload theme” button
7. Upload and install the child theme zip file
8. Activate the child theme