This guide assumes you've already installed WordPress on your server, if you haven't done that yet please refer to the WordPress installation guide here first and then come back to this guide.
Go to your Themeforest downloads page, click on the green Download button and select Installable WordPress file only https://themeforest.net/downloads.
Login to WordPress, go to Appearance > Themes, click Add New, upload the zip file you downloaded in step 1, then click Activate
You'll then be prompted to install the required plugins, click on the Begin installing plugins button
Next click on Demo Import and click through all the links to install the entire demo website
And you're done, your website should now match the demo website. However if that isn't the case and you've run into some problems, please open a support ticket and I'll be happy to resolve any issues for you.