The one I use is made by Kaiser. It's the sort where you slide the larger "tongue" into the cassette, then turn the film spool until you hear a faint click before sliding the second smaller tongue in from the retriever. Then just turn the spool the other way until you feel resistance (trapping the leader firmly between the two tongues) and gently pull the retriever out. Works most of the time, I've never had to have more than two attempts on a given film.