OK, so here's my land skirmish mapping tutorial
Step 1
Start the map editor. You should see a screen like this:

Step 2
Press the 'new' button in the top left hand corner. You should see this:

Step 3
Change the settings to Map Type: Arctic
Faction Owner: Branched
Player Mode: Multiplayer
Players: 4
Map Levels: 5
Map name: My First map
Planet Name: Hoth (No spelling mistakes)

Press OK
Step 4
That dirt looks boring, so click on the Texture tab (Called 'texture' with a little spray brush) and press load in the window on the left (see image)

A box will appear with a selection of texture sets. Double-click on Default_Arctic.

Step 5
Now there's snow everywhere. That corner of the map you are in is a bit boring, so hold down the right mouse button in the 3D window and drag around to move the view until it is the same as the one below.

Step 6
That map is a bit flat, so click on the 'Height' button. In the settings section of the left window, change:
Radius: 150
Intensity: 10

Now you can make some hills.
Step 7
To make a hill, just click and hold the left mouse button. The terrain rises, and you get a small hill, depending on your radius and intensity settings. Play around for a bit making hills so you can get the hang of it.

Step 8
Those hills are a bit pointy, so set smooth weight to 0.7 and hold the middle mouse button over a hill. It should flatten out.

Step 9
A river would look nice, so set the Radius to 100 and hold done the Ctrl key. This will make the ground lower, and form a river.

Step 10
Those yellow lines are the edges of the map. That means that no units can cross them, except bombers, and the camera cannot cross the line. To make the playable area larger, click File>Map Properties, then adjust the Top, Bottom, Left and Right Bound sizes to 100 for each.

Step 11
The water in the river is much too blue. To change this, and make the sky look better, click on the 'Nature' tab.

Click on the Load button in the left window (it's at the bottom) and double-click on Default_Arctic. You should see everything go white.

To add a sky to your map, use the drop down Dome1 menu to select Day_Snow. This will make a cloudy sky appear.

To move the camera into a position similar to in the image, hold down the Cntrl key when you are moving around with the right mouse button. The view will rotate. To zoom in and out, scroll the scroll wheel of your mouse fowards and backwards.
Step 12
I'm sure you're dying to know how to add structures to your map, so here's how to do it. Click on the Objects tab. Scroll down in the left hand window until you find a 'Multiplayerstructuremarkers' section. Click the little plus button next to it, and you should see a list of buildings.

Add one of each building in the Team00 category. Place them where you like, but try to put them in strategic positions. There's no need to add build pads yet.
To place the buildings, select them in the left hand objects window and then move the mouse into the 3D view area. Hold down the shift key, and the object will appear. Click to place it. To modify the angle that the building points in, change the Z angle value in the building's settings window.

The command post needs to be placed too. Put it on using the same method as for the buildings.

Step 13
Now would be a good time to save your work. Use the floppy disk icon in the top left and save it as whatever you want.

Step 14
To add build pads to your base, select team00_buildpadsposition, and place them on your map as you would any other building.
IMPORTANT: to get build pads to point in the right direction, make sure the two dots highlighted on the image are pointed in that direction.
Back of build pad

Front of build pad
Step 15
Now do the same with Team01's base (Team 2 in the game). Place all the buildings using the shift key and left mouse button as before. Don't forget the build pad pointing.

Step 16
All the same type of snow looks boring, so click on the Textures tab again and mess around wiht painting textures until you get one you like. To paint a texture, click on the texture you want to paint in the little box of thumbnails, then click on the map in the 3D view area. To change the size of the brush, use the Radius setting in the bottom left. I'd suggest setting it to 50-100.

Step 17
Now you have a workable map, test it in-game. Run FoC and select Custom maps in the One palyer Skirmish menu. Your map should be there. Test it by having a battle. You may notice your troops wading neck deep in the water and AT-ATs climbing mountains, so you know you need to change passability. To do this, start the map editor again and load up your map. Click on the height tab. In the Brush Style drop down box (top left) click on Passability Code Brush. Everything will go green and purple.

To chnage the passability of hills, set Radius to 100 and set Passabilty to steep. Paint the hills like you would with a texture. They should turn red. This stops all units except flying units from going over the mountains.

Don't foget to go all the way round the mountains.
To stop the stormtroopers getting their feet wet, select water passability and paint the river. This only allows repulsorlift vehicles to pass.

To go back to normal views, change the Brush Style to Additive Code Brush.
That just about ends this tutorial. For more advanced land editing, see my land GC map editing tutorial. Please feel free to comment or ask questions.