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.
When using authentication with Maskinporten (link in norwegian only), you must configure the requests in postman for Bearer token. This can also be combined with the MaskinportenTokenGenerator utility, which can be used to retrieve and set the bearer token on the request through a pre-request script.