How do I add Schema.org or JSON-LD snippet markup?

To generate the code of the snippet markup, use a generator such as the Ultimate JSON-LD Generator: https://webcode.tools/json-ld-generator

Go to the generator website and choose a page type – Article, for example.

Fill in the required fields. They will differ depending on the page type you've chosen. Copy the code in the column on the right.

Go to Tilda, open Page settings → Additional → HTML Code For Head-Zone.

Add the code. Save changes, republish the page and wait for the next indexing cycle.

To check if the markup is correct, use the snippet markup validators. Both Google and Yandex have tools for testing structured data. Use them to test and fix your code.