The editor runs as a native Windows 95 application, so you can resize it fullscreen for editing. The screen is split into a window on the game map, plus two mini windows, one of which contains units you can place, the other having terrain you can add. Editing the map this way reinforces the fact that Command and Conquer actually has only one terrain level, though clever use of terrain tiles can give the illusion of cliff tops. Placing all manner of objects, including Tiberium fields, neutral villages, forests, roads, rivers, beaches, barbed wire fences, etc, etc is no problem. The only minor complaint is that on a 1024x768 screen the tiles are quite small, but that's not really the author's fault.
Triggers are events which can spark actions, eg.
Or if a certain building is destroyed, you could remove a certain trigger. What you can achieve here is quite sophisticated, but Michal Parker's otherwise excellent manual is a bit lacking on this side of things. While not all scenarios need teams, the more imaginative use you make of them, such as assigning movement waypoints, the better the scenario will likely be. Hopefully more info on teams will be in future versions of the manual. When you've finished editing you can use MixMan to insert your mission back into your hard drive copy of the game campaign file, then run it by running the game and restarting from a saved game matching the scenario you replaced.