The only way to Herm is by boat from St Peter Port, Guernsey. The Travel Trident ferry runs the 20-minute crossing several times a day from Easter to October, less frequently in winter. Tickets are sold at the Guernsey harbour office and aboard.
Sailings depend on the tide — Herm's harbour dries out at low water and the bigger ferries divert to the Rosaire steps on the south coast instead, a longer walk into the village. Check the schedule.
There is no airport on Herm and no cars. From the harbour you walk everywhere. Day-trippers normally catch the last ferry back around 5 or 6 pm; staying overnight requires booking the small White House Hotel or one of the self-catering cottages well in advance for summer.