You've installed Blades of Avernum correctly, played through all the scenarios that come with it, and installed the Editor. By now, the urge to play around with the Editor must be irresistable. If that's what you want to do, go ahead. Open a preinstalled scenario and start editing to your heart's content (as long as you don't save, you won't break the scenarios). It's a good way to get familiar with the Editor, but it's a bad way to start designing a new scenario. It turns out that you can't start making random dungeons and filling them with random monsters, and have it magically turn into a scenario (let alone a good scenario). Let's start planning the scenario you'll be making for the tutorial by asking ourselves a bunch of questions.

Whew. Now that you've got a working plot, you can finally open up the Editor and create the scenario.