Leading up to clearing, pages on each uni's website will appear which are dedicated to clearing, and they'll show which courses have vacancies and what numbers to call.
I wouldn't bother making a list until the clearing lists are up though, you don't know which unis are going to have spaces left. The clearing list appears the night before results day (at least it did when I applied a few years ago), make your list then, with all the numbers on it - that's what I did, and I just went off the list when it came to it.