Build a breadcrumb trail that holds together
Ordered ListItems, meaningful names and the final-item exception.
Describe the page hierarchy
A BreadcrumbList represents a meaningful path through a site. Google’s documented profile uses at least two ListItem entries. Give each entry a position, starting at one, and a readable name.
Link each step
Earlier entries need an item pointing to the corresponding page. The last entry may omit item; Google can use the containing page’s URL. A Thing used as item can provide the name instead of duplicating it on ListItem.
Validate the whole trail
CheckMySchema checks entry types, sequence, minimum length and required fields. Fix the hierarchy itself before fixing syntax if your breadcrumb does not accurately represent navigation.
Go deeper with the official sources
Google breadcrumb guideBreadcrumbList definitionThis is an original explanatory guide, not an official search-engine document. Requirements can change. Follow the linked sources for full, current details.