You are usually given 2 ratchet straps per bike to secure it. But be quick as some of the foreign riders take 3 or 4!
Leave it on the sidestand if you can and lock the steering
Depending what bike you got you can usually pass one strap through over the top of swing arm pivot just behind the rear wheel and in front of the shock absorber. When this one single strap is tensioned up the bike is securely planted to the deck for the trip.
If your really worried fix another strap but make sure it's somewhere safe
Also take some old rags and put them between strap and bike, save scratches
Depending if you are on the left hand or right hand car decks there is a cable running down the outside wall of the ferry which you can clip your strap onto.
It is important to line up the fixing eye on the deck with your swing arm position so that when the strap passes through, it is more or less straight across the bike.
HTH