It’s easy to install Once. Just follow these steps, they won’t take much of your time.
Go to ThemeForest Downloads and next to Once, click the Download button. There’re two options to download Once: All files & documentation or Installable WordPress file only.
If you select the Installable WordPress file only option, the downloaded archive is the theme archive file you need to upload to your website.
If you select All files and documentation, you will need to extract the downloaded file on your computer. Inside the folder you will find another archive called once.zip. This is the theme archive file you need to upload.
Login to your WordPress admin panel and navigate to Appearance → Themes.
Click Add New button at the top of the page, then the Upload Theme button on the next page.
Select the theme archive file on your computer and click Install Now.
Once the file is uploaded and installed, click Activate.
Go to Appearance → Theme License and input your purchase code. You may find the purchase code in your ThemeForest account, please refer to this document for more details.