The easy answer to your question is: There is no such thing as an order when you have to use MED, WED, or SED, or when you have to create a terrain and so on. That's probably the reason why you haven't found any information on this topic, yet.
You can either start with the level design or with a gameplay prototype, and I suggest you start with the latter. Think of a game, then take a few notes (or even plan it out on paper), then create a simple dummy level with WED that features a few certain elements (f.i. stairs, slopes, doors) and a temporary player model. Now use SED to prototype your gameplay respectively your movement first and test it in the dummy level you've created previously. If all seems to work out fine and if it is fun, you can start to create nice levels with your favourite program (and import via FBX) or use WED directly (that's what I do). You could even write your own editor but that is not necessary.
In MED you create or import your models, your player figure, enemies, other animated stuff, detailed objects and the like. When you come to a point in your level where you say: "I need a terrain, now!", you can start MED and create a terrain there, too, manually or via a heightmap. The advantage of a heightmap is that you can create your blendmap in a painting program, too, otherwise it can get difficult (I've posted a very simple and unfinished blendmap painting tool called TED on the forum some time ago, it may be useful for you).
I don't want to blame the developer but simply never use GED, it was an approach to create a realtime editor that uses the engine without having to rewrite anything, that's why you cannot edit blocks and the like in it. Everytime I used it I've only had problems and crashes.