Travelling to Unst or Fetlar involves first catching the ferry from Mainland to Yell, before crossing Yell by the high quality road running to the terminus for the Bluemull Sound ferry at Gutcher. For current timetable and fare information, and for bookings, visit the ferry area of Shetland Islands Council's website.
Shetland Islands Council suggest you allow 25 minutes to cover the 18 miles across Yell. Given the quality of the road that's realistic in good weather, but allows little time to do justice to the attractions of Yell.
The crossing from Gutcher on Yell to Belmont on Unst takes around 10 minutes. It is, however, part of a triangular service (the "Bluemull Sound" service) which also involves the rather longer, but less frequent, crossing to the new ferry rerminus at Hamars Ness on Fetlar. As a result, though the services between Yell and Unst are at approximately half-hourly frequency from fairly early in the morning until late at night, they are a little irregular. The crossing from Fetlar to Yell or Unst takes about 20 minutes and operates on a "several times a day" basis.
Booking is probably a good idea for all services during the peak season. Fares are very reasonable by standards elsewhere in Scotland.
Two vessels operate the Yell-Unst-Fetlar service at any given time. The larger and prettier is Fivla, which comes complete with a well heated passenger cabin under the vehicle deck. The second is either Fylga or Thora, both of which are rather smaller, though still perfectly capable of carrying the large HGVs that often use this service.
At Gutcher the waiting room is supplemented by the excellent Wind Dog Cafe, overlooking the pier and loading ramp. Also nearby is a post office that doubles as a B&B.