what i do is for a get call;
1 - create json structure (paste an example json response)
2 - create import mapping (select elements - json structure: see 1 - check all
3 - make the mf that has a call rest api -> fill in the call rest activity details....choose in response; import mapping for entire response: mapping of point 2..
4 - make an error handler to if the calls failes to check the stacktrace.