I've just installed, and the default setting is great. But  wading through all the help files, I can't find an answer to the question that bugs me: once I've created my own ground and sky texture sets, what then? When I launch  Fs9, where and how do I identify and select the sets I want to use?

You can make your own ground textures if you want but you have to chose which ones you want FS to use as you can not select them at the start of each flight ( But if you make new ground for say a airport then you can either use the one you made or the default one. But you have to replace the original file with your file before starting FS. You can not just pick the ground you want at the startup of FS.
If you use something like GE PRO it will pick the ground according to the season .

Fs by default already does that but some do not like the textures it uses.

Thanks, everyone. Your replies much appreciated.  I should have explained that I'm a technodummy, because frankly I'm still confused. As a low and slow pilot in both real and virtual worlds, I never fly long enough or far enough to make use of the GE PRO weather change facility. At present I have only the default setting, but I'm very happy with that. So the question is simply whether, for my kind of flying, there's any point in installing any of the many available ground and sky texture sets, since they seem to be geared solely to distance flights. If I'm wrong, then I still don't know if or how I could make use of them for, say, an hour's local flight arouind Seattle...

You are a true Simmerknight, Sir, and I'm much obliged. It does indeed make lots of sense, and is exactly what I needed to know.