Easy when you know how - but it'll take you a few hours/days to find out.
Get FSSC (flight simulator scenery creator) and EOD (easy object designer) for starters. Both are freeware. Do the tutes in both (no-one can TELL you how to do it - you've got to put the work in yourself).
Within a coupla hours you'll get the idea and you'll have your first 'rough' scenery in place that I'll guarantee will amaze you and of which you'll (rightly!) be very proud.
Then you'll develop it until you get something like what you're after. For textures in the beginning you can use the hundreds that come free in your main FS2002\texture folder. You'll find stuff that'll give you what you need - for starters. If you start to make more custom buildings in EOD you'll need to find out a bit about texture theory - difference between normal and extended bmps, how to get transparency and so on. Then you mught need some help and advice here. Plus you'll need a good graphics program - I use CorelDraw for example - and a 'painting' prog like PaintShop Pro.
If it grabs you I can guarantee you'l find scenery design a VERY exciting and satisfying road to go down.
Roger