I do Hull to Zeebrugge quite often. You'll be directed to the bike carrying section of the ferry where there will be two wire ropes secured to the floor. You park your bike in the middle of them, put it on the side stand and leave it in first gear. You will be given a strap with a hook each end and a buckle type thing in the middle. Secure one end to the wire rope one side of your bike, take it over the saddle and secure the other end to the wire rope on the other side of your bike. You then put weight onto the saddle to compress the suspension and pull the free end of the strap tight. The buckle is an interference jobbie which holds the strap tight, and your bike secure to the floor.
Don't forget to take your bike out of neutral in the morning though, I've seen people sat wondering why their bike won't start while everyone behind them is trying to get past !