Shopify installation

Open Online Store → Themes. On your current theme, open … → Edit code.
Open layout/theme.liquid. Paste the snippet just before </body>, then click Save.
Open your storefront. If password protection is enabled, remove it before checking installation.

Open Setup for your complete, ready-to-copy snippet.

After publishing, click Check my install in Setup. See troubleshooting if the assistant does not appear.