To get postman, just follow this link:
Using Postman is very simple. Just click on new, then, create requests. Then, you can configure them: change payload, add header, ... Then, click on send, and the request will be done.
The interface is quite clear. Request is on top, response is at the bottom.
Sometimes, it's quite useful to have the ability to replay code we've observed in our browser, for instance in Google Chrome. This could be the case when the call is very complex (with a large payload, some cookies to set, or anything else).
In that case, just go to the network inspector tab, right click on a request and copy as CURL.
After that, you can copy it in Postman and replay the call.
Click on import
Then, select "raw text"