Install Postman (https://www.getpostman.com/). Signing up for an account is not necessary, you can go straight to the app.
In the top left corner is an import button. Either drag the Altinn.postman_collection.json file from postman-examples/Collection into the window or click choose files and select it.
In the top right corner there is a button with a cog wheel called manage environments. Click it, then click the import button, then click choose files. Select all the json-files in postman-examples/environments/ and click open.
For more information, go to https://altinn.github.io/docs/api/rest/
When using authentication with enterprise certificates (link in norwegian only), you must configure a client certificate in Postman. For testing in dev environment without TLS-termination, you must supply the certificate in Base64 PEM format (without BEGIN/END headers) in a HTTP header named X-ENV-SSL_CLIENT_CERTIFICATE for authorization to take place.
For all non-dev environments, all requests must include the query parameter ForceEIAuthentication in order for TLS client authentication to take place. This is already included in all service owner operations.