Pirogues to the islands leave from the beach near the old Novotel in central Conakry. The crossing to Soro on Kassa takes about 30 to 45 minutes depending on the swell and the size of the boat.
You can either join a shared pirogue -- cheap, but you wait until it fills up -- or hire a private one for a few thousand francs. Agree on the price and a pick-up time before you leave: there is no scheduled return service, and being stranded on a small island after dark with no booked boat is uncomfortable at best. Bring drinking water, sun protection, and small notes for food on the islands.