In order to create BreadcrumbList markup, you'll first need to create a BreadcrumbList data item.
Ensure you've provided a name for the data item, and that the URI matches the URL of the page you're creating markup for.
The BreadcrumbList is created by connecting the "itemListElement" property to as many ListItem data items as there are breadcrumbs. To add ListItems, click into the "itemListElement" property, and click "Add Data Item...". Ensure the type is set to "ListItem"and provide a name. The name should match the name of the Breadcrumb as it appears on the live page. Then click "Create".
Next, go into the ListItem data item you created. Provide a numerical value for the "position". If it's the first breadcrumb in the list, it will have "1" as the value. Then, copy and paste the URL associated with the breadcrumb into the "item" property:
Once you've add all of your ListItems to the Breadcrumb List data item, test your markup using Google's testing tools to verify your work. Here's an example with two ListItems contained within a BreadcrumbList data item:
For more information about implementing BreadcumbList markup, check out Google's documentation on the subject at: