If you've already prepared your save file, you can skip to step 13. Part I - Accessing the Custom Scenario Menu in SCE: * - This tutorial will focus on custom scenarios, but similiar instructions can be followed for Crusade Extreme.
It is very possible to make custom scenarios and free build maps directly in Stronghold Crusader Extreme this post will teach you how to both play and create scenarios for Stronghold Crusader Extreme (henceforth SCE or Extreme).*