Firs thing you need to do is find the waterline on your boat when you are on plane. This is important because you want the transducer to maintain contact with the water all the time in order to get a reading. The faster you go the less accurate the sonar will be come. However when crusing around it should always give you a bottom depth reading. This is essential in keeping you and you boat safe, by makig you aware of sudden depth changes.
Second thing you will need is a transducer mounting bracket. Follow this link:
http://m.cabelas.com/cabelas/product...6447&id=701430
It attaches to your hull with 2 screws. The transducer is than mounted to this bracket. Saving your hull from a whack load of screw holes. This also allows you to make adjustments until you get things set just right. I would also recommend applying 3m 5200 marine sealant to the back side of that bracket. This sealant will go rock hard and is designed for application below the water line. I can testify to its ability as I purchased a used boat this year and removed all the old brackets off the transom. Gob of 5200 plus a screw to plug the hole. Doesn't leak a drop.
On a final note and if my memory serves me correct you can order from cabelas.com and they will ship for free to any Cabelas store. Hope this helps!