In order to install it you have to simply remove the steering cable from the rudder, then remove the nut, and install the new one. I prefer marine grease over the oil they recommend. Make sure to grease your rudder and the steering cable mount when you are back there.
It appears you can also buy just the nut, so you do not have a buy a complete kit. You can also buy two nuts in a kit for a discount if you wanted to share with your buddy.
The one that fits your boat is the one that fits your steering cable. If you read the specs the nuts are pretty standardized. Check the size and pitch of yours for ordering. Seems like most inboards use the same nut though.
Here is their full marine catalog. http://www.davisnet.com/product_documents/marine/CatalogsFlyers/Marine_Catalog.PDF