After purchased item from the themeforest, you can visit: https://themeforest.net/downloads to find your downloadable themes.
Once you downloaded and save your zipped folder somewhere, extract its content then you might see something like this:
- Help.zip will contains this help document once extracted
- Antive by Fami_themes.zip is your upload able content to the Shopify store
Now go back to your store admin, navigate to Online Store\Themes Then choose to upload theme
Find and choose to select the file Antive by Fami_themes.zip which you already extracted before
Click on Upload and wait for the process to be finished.
Now you can either publish your new installed theme or just simply customizing it before publish.