Okay I have figured out how to use most of the basic features of the map editor after playing around with it for a few hours yesterday and I have decided to post up a very simple guide for beginners on how to start making their own maps. It should hopefully save you a few hours of your time.
Step 1: Change the camera settings
Go to the right hand side of the Editor window and you will see a tab called "tools" which has already been highlighted. Double click on the "Camera" option and a panel will open up beneath the tab. Scroll down that panel until you see the movement mode option and change that from "look" to "orbit". This will allow you to view the map from above, which in my view is the easiest way to use the editor.
Don't change any of the other camera settings. To move the camera use the right click and the mouse wheel or the WASD keys. You'll eventually figure out how move the camera in a way that suits you.
Step 2: Deployment Zones
Look at the top left hand side of the Editor window and you will see a window that says "create". Click that, then click the option that says "object type" and change it from "instance brush" to "deployment zones". You will need to make two different deployment zones for the opposing sides.
First, select "alliance id" and then hit "0" if this is not already selected then make a deployment zone by left clicking and then dragging the deployment line as far as you want it to go. Left click again to set the deployment zone. Then set the deployment zone for the other team by setting the alliance id to "1" and repeating the same action.
If you're a bit too lazy to mark in the deployment zones, just select "file" in the top left corner and "new map" and the editor will generate a flat map with two marked deployment zones for you.
If for whatever reason you mess up a deployment zone hit "ctrl + z" and it will remove the deployment line.
Step 3: Change the type of ground to suit the terrain that you intend to add.
This is very important, as the ground type affects certain units and their special abilities. I am finding that trees that are not placed on top of forested ground can't be used to hide your troops.
To change the ground type, select the "paint" tab in the top left corner and set "blend" as the paint channel. Then change the blend options to suit you - I personally only change the ground type and the scale (which affects the area that the the brush changes). Use the left click button on your mouse to pain the selected terrain.
I start by selecting "ground type forest" and setting the "scale" to about 500 and marking the ground where I want to put forests on the map. I will then switch the ground type to "Mud" to mark in paths, roads (I keep the scale on 10 when I do this - this seems to be enough to mark a road)and to make the areas below villages and settlements look like they have been worn down by the people living in them.
Step 4: Add high/low ground.
Switch the paint channel to "height" to add mountains or depressions in the ground. You can change the size of these by altering the "scale" option and the height using the "height" feature (values greater than 0 will increase the height of a piece of land, values of less than zero will decrease it).
Step 5: Add trees
Change the "object type" to "Vegetation" then hit the "select trees" feature on the right hand side of the window. This will open a window that gives you a list of tree types to choose from - once you have found the tree type that you want hit "OK".
Then, change the mode - "single" will place a single tree, whereas "brush" will place a group of trees. You can change the "scale" to change the size of the area that you wish to cover with trees when you are using the "brush" mode.
Step 6: Add Buildings
Change the "Object type" to buildings, then click on the "model selection" to change the type of building that you want to place. Don't forget to put mud below any built up areas, otherwise they will look like they've been deserted.
Step 7: Add Capture Points.
Change the "Object type" to "capture location" and you will be able to place capture points by left clicking.
1) When moving or clicking on terrain features, always make sure that you have the "select" feature in the top left corner enabled, otherwise you'll drop terrain and markers that you don't want.
2) Save frequently. The editor can crash on occasion and its a pain to have to start from scratch.
3) When saving make sure that you select the right save file. The editor lists saves in order of "first to last" rather than the other way around so its easy to overwrite previous maps by accident by clicking and not watching what you're doing.
4) "Ctrl + z" will "undo" any recently created map features (hills, deployment zones, buildings - everything). You will use this often although be careful to just remove one terrain feature at a time, don't hold down the button as this can crash the editor.
5) "Delete" will delete any terrain feature that you select. This is obviously very useful.
Disclaimer: I am still a complete newb at this, so I can't answer any really detailed questions - direct those at the CA staff. This guide is only meant to help you through the first few hours that you use the editor, the rest you will have to figure out for yourselves.