I just did this to starboard side of my boat, port next. I just did the bottom the sides and top are still okay. I was able to remove the old by prying it out with a need nose pliers and a screwdriver (it broke apart it was brittle)
Obviously sliding the window back and for to get it out from both sides.. I cleaned the tracks with a hose and scotchbrite. I then cut the new material about 1 in larger than needed . I also cut in the drainage slots/hole to match the aluminum slots I tide a string to one end of the new material with a small hole in it. then I lubed up the window, the tracks and the new weather stripping with soap water and I was able to pull it under the sliding windows. By pulling the string and sliding the window back and forth. I have done 4 windows so far, it easier than it sounds with the help of
window installation toronto.
Good luck
Ron