Do you mean "Onshore trips" ? If so either through the official OV web-site, or by ringing their head office in Southampton or do what most people do and wait until you are onboard and choose there. Neil Dow.
You can either wait till you are on board, when you will be given a list of all the excursions and costs, or as Down, mentioned you could book before you go. But be a devil and venture out on your own, there are plenty of taxis at each port and each night you will be issued with the ships news paper which will adivse you of the local attractions and where to catch trains and other types of transport, we used the arranged trips, but wished we had gone it alone, we spent around £400 for a weeks worth of trips, when there were several couples who only spent half that.