First please install and activate Elementor and Starto Theme Elements for Elementor plugins before you proceed.
Once you get required plugins installed. Please follow below steps.
Create a page or edit an existing one.
Click “Edit with Elementor” button to open Elementor Page Builder.
Click “Add Template” button and scroll through “Pages” section.
You can filter only Starto templates and select demo page you want to import and click “Insert”.
If you have experience Starto template issue with Elementor library. Please make sure you update the plugin “Starto Theme Elements for Elementor” to v2.0