'Awkward' is a charitable word to describe the Scenario Editor

.
While it
is quite powerful, I don't believe it ever got 'polished' to the point of even approaching the sacred status of 'idiot-proof', so it is still quite 'dangerous' when it comes to everyday use, so 'don't do this at home'; do it somewhere else, like at work, where profanity and monitor smashing are acceptable.
In other words, save frequently.
Aside from the occasional abrupt exit, the ScenEd does not always accurately mimic the game's behavior, but this is mostly misleading rather than really critical - things like ecodamage or the analysis of tile production - probably due as much to last minute changes in the game as anything. I have also managed to somehow get scenarios into seemingly terminally effed up condition, but I don't know for sure whether or not it was my fault (as this has diminished over time) or some fault in the ScenEd, so I usually give new names to my save files most of the time I save, so that I will have a clean copy to fall back on. Practice is pretty important, as eventually one will get a feel for what kinds of things are OK and what is risky or self-destructive.
Googlie has prepared some useful help on the subject, available
here, which covers nearly all of the essentials, but the learning curve is not so easy and there are still things left unsaid and I'm sure there are still plenty of important aspects I am completely unaware of.
(For a long time I mostly overlooked many of the extra buttons the ScenEd inserts into otherwise familiar screens, some of which perform very useful services.)
However, after a while, one gets used to the ScenEd and it is possible (only slightly reluctantly) to give it a rating of somewhere between 'not bad' and 'not too bad'; it is, after all, capable of doing its job more than most of the time. I suspect that its original purpose might have been more as a development/testing aid than as a consumer product and when viewed in that light, it is really quite a nice tool.