Scott - It's easy, just read and follow John Foster's article under Grab Bag, Trailer, Bunk Replacement! I did mine last year. After buying the materials it took about 1 hour to cut and staple the carpet, measure and pre-drill the pilot holes, and then about 3 mins once I launched and beached my boat to replace them on the trailer.
If you do use the new treated wood, you have to be careful!
The new chemicals will eat right through the old deck screws. You need stainless steel or ceramic coated nails or screws. I was told that if the fastners were galvanized, if they were double dipped it would be ok ?
Rick - I just checked, I made a mistake up above, what I used were "Monel Rustproof Staples" they are made of 66% nickel and 33% copper, just checked them on the trailer, and their in perfect shape. Got them at home depot.