Browse our themes at /themes, pick one, and complete checkout. You'll receive a license key by email and it will appear in your dashboard.
Go to your Dashboard, find your license, and click Download. You'll get a .zip file.
In your Shopify admin go to Online Store → Themes → Add theme → Upload zip file. Select the downloaded ZIP and click Upload.
Once uploaded, click Customize to configure or Publish to make it live.