JSON-LD is the recommended format for writing structured data: a JSON object placed in a <script type="application/ld+json"> tag, with @context pointing to schema.org and @type specifying the entity. It is separate from the visible HTML, and its content must match what the page actually displays.
Example
<script type="application/ld+json">
{"@context":"https://schema.org","@type":"Article","headline":"Homemade Bread"}
</script>