How can I manually find the prototypes of native APIs ?

Please don't post some link or some book's name (like Gary Nebbet's book) listing the prototypes of the native APIs.
What I want is how can I reverse engineer the ntdll.dll and ntoskrnl.exe to find the prototype myself.(I have fairly good knowledge of reverse engineering so you can use the jargon associated with that).

How do I know what is sth (is it a window handle or pointer to a string or whatever) and sth_else ?

well, download IDA, open NTDLL.DLL, look at exported procedures, figure out how many arguments procedure thakes and what they mean (using your reversing skills), and there you go.