Installing a Bath Fan Roof Vent Without Attic Access?
You say "no attic". That can mean one of two possibilities:


Bathroom ceiling is very close to the roof


In this case, you should cut the hole in the ceiling for mounting the new fan and simply uses a reasonably long drill bit like this 12" drill bit to drill a pilot hole, then go on the roof to drill a larger hole around it. In this case, there won't be much ductwork needed and the installation should be very easy.


Bathroom ceiling is far from the roof


You could easily have a few feet between ceiling and roof without having a functional (and accessible) attic. In which case you need a really long drill bit like this 72" (6 feet) one to drill a pilot hole, then go on the roof to drill a larger hole around it. The problem then will be the ductwork. With a little luck, and a bit of work, you may be able to get rigid ductwork lined up just right and drop in from the top and get it over the fan location. But in that situation it may be a lot easier to use flexible ductwork.