You sail to Lochmaddy, tour North Uist by coach, then get the short ferry crossing to Harris and return via the main ferry from Tarbert to Uig. On Tuesdays, Thursday and Saturdays you can do the journey in the opposite direction by sailing first to Tarbert, touring Harris by coach, then getting the small ferry to North Uist and returning via Lochmaddy. Departures from ferry terminal Uig on Skye at 9.45am, return at 5.35pm. Sundays only from 27 May to 2 September 2001. You can pick up free sheets with timetables for each local route from tourist offices in the Western Isles and at Stornoway bus station.
Buses generally run in time with the ferries and there are less in winter than in summer.
My travellers' tips file may also be useful if you are planning a trip to Scotland. It covers airlines, national car hire, transport information, maps, etc.