Hi,
At OR2021 conference earlier this year, we had a "Getting Started with DSpace 7.0" workshop which covered some of the basics of the REST API. Those slides & video can be found at
https://wiki.lyrasis.org/display/DSPACE/DSpace+7+at+OR2021
It's also possible to learn some of the basics of the REST API by simply watching the requests our DSpace 7 User Interface makes to the REST API. Every page of the UI is built via REST API requests, and all those requests are easily visible in your web browsers "DevTools". These exist in every browser, but in Chrome, you'd go to "More Tools" -> Dev Tools, then open the "Network" tab. At that point just click on pages or buttons in the User Interface, and you'll see in that Network tab every request made to the REST API.
At this time, we do not have an updated step-by-step tutorial available, but those resources should provide you with a place to at least get started.
If you have other specific questions, let us know on this list, or on our dspace-tech Tech Support list.
Tim