This is a very simple Postman collection that illustrates how to get a CSR signed by a step-ca server.
First, in Postman's general settings, under Certificates, add your CA Certificate PEM file:
Now import the step-ca Postman collection.
In the collection, go to the Variables tab, update the value of the caUrl variable to match your CA endpoint, and save the collection.
Now run the "CA Health" request. You should get a "ok" response from the CA.
To get a CSR signed, you will need to fill in the values for csr and ott in the request for the /1.0/sign endpoint. The easiest way to generate those values is to use one of the other clients in this repository. Find more details in the Postman documentation for the signing endpoint.