Changed mine for the first time about 4 months ago and just watched a video on youtube. I have a Yamaha 70 2 stroke, but I wouldn't imagine the impeller/water pump would be any different for your four-stroke. I just took my time and took a picture of every step that I did (as a fail safe).
While you have the lower unit dropped, take the time and really look at everything. Grease all the splines and other areas that need grease. While the prop is off, go ahead and regrease the prop shaft too!
https://www.youtube.com/watch?v=W3sr6El4rh0
This is the video that I followed, but did have a couple different gaskets than his, but all in all, very easy to do.