The best way to really see this WHS is to travel along the river (although I did see a few people cycling the roads along the bank - also a good option).
I think many people go on organised cruises but you can also just get the ferries in between the different towns and explore on your own. I used the KD ship - started in Bacharach and finished at Koblenz, stopping at Sankt Goar for an hour along the way.
I stayed the night in Bacharach beforehand which was a really lovely spot. I used the youth hostel which is in an old castle at the top of the hill - quite spectacular accommodation!