First create a type in the database that describes the collection that we want to open the ref cursor for:

create type num_tbl is table of number

Then create a function that will define and populate a nested table of this type, open a ref cursor for it and return that ‘handle’:
create or replace

function get_num_cursor
return sys_refcursor
is
  l_num_tbl num_tbl;
  l_idx integer;
  l_refcursor sys_refcursor;
begin
  l_num_tbl:= num_tbl(10,20);
  l_num_tbl.extend(5);
  l_num_tbl(l_num_tbl.last):= 30;
  l_num_tbl.extend;
  l_num_tbl(l_num_tbl.last):= 40;
  open l_refcursor for select * from table(cast (l_num_tbl as num_tbl));
  return l_refcursor;
end;

Source