Be sure to have a working OpenCart 3 installation before you attempt to install the theme.
1Copy theme files
Unzip the theme archive you have downloaded from ThemeForest. Upload all the files from OpenCart_v.3.x folder to the directory you have installed OpenCart in:
In OpenCart admin panel go to Extensions › Themes and install the BurnEngine theme.
You can now check the First Steps section in order to find out how to install some sample data or adjust the theme settings to your needs.