I have played a couple of the unofficial single player missions made by fans around the world.
I liked the war trilogy by Freshmeat and the 7 Gates to hell by David Paularena.
So naturally I thought: "I want to be able to do that".

Well let me tell you this...
If you want to make your own single player missions, then you need:

• To read the RVMech tutorial
  and the EA games World Builder tutorial
  - (as I will not cover the basics of building the landscape)

  NOTE I was stupid enough to mirror RVMechs tutorial without asking him for permission, that was dumb.
  I apologize, for he has done some great work with that tutorial - so basically - sorry! : (

• To put this link in your "Favourites"
  
  - (because it is the place to ask your mapmaking questions)
• Stubbornness
  - (it will keep you trying to figure out why your scripts aren't working)
• Lotsa time
  - (it takes a lot of time to make a good single player mission)

...so if you meet the above requirements, and have read the other tutorials, then by all means continue : )