When working with the API, you will find two categories of objects: platform-level objects and instance-level objects. When working with instance-level objects, you must pass the special header teal-instance-id, which should contain the Instance ID to be accessed when making the request. For example, here is a request to get the ledgers for an instance: 
curl --request get \
--url 'https://api.sandbox.teal.dev/v0/ledgers' \
--header 'Authorization: Bearer <token>' \
--header 'teal-instance-id: <instance_id>'