Advance preparations
I would add to what Steve said by some advance preparations, because once you start the process, you want to be able to finish it since you may have some water coming in. First, you need to determine the correct size of packing that you'll need, because it's critical that it fits properly. (If you don't know already, you can back off the lock nut and packing nut and measure the space between the shaft and the edge of the packing nut where the packing will be inserted.) Then I would pre-cut the 3-4 pieces of packing that you'll use. My prop shaft is 1 inch dia, and I took a piece of dowel rod the same size and used it to measure and cut the packing. Use a sharp utility knife, and make a diagonal cut so that the ends just meet around the shaft. Depending upon the packing you use, there may be a lubricant/grease that comes with it to coat the packing which makes it easier to insert. Good luck.Good luck